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Rungwise

Practice bank

Every practice problem from every lesson, in one place. Filter to find your next problem. Each one opens in its lesson, with hints and worked answers.

Difficulty

259 problems from 82 lessons.

  1. Counters that don’t shareEasy

    Python · from Every button I made in a loop says the last name · Tests run here

    Topics: Closures

  2. Remember the answersMedium

    Python · from Every button I made in a loop says the last name · Tests run here

    Topics: Closures

  3. An editor with undoHard

    Python · from Every button I made in a loop says the last name · Tests run here

    Topics: Closures

  4. Keep the last linesEasy

    Python · from My import got 100 times slower when the data grew 10 times · Tests run here

    Topics: list, tuple, dict and set, and what each operation costs

  5. Add up two days of salesEasy

    Python · from My import got 100 times slower when the data grew 10 times · Tests run here

    Topics: list, tuple, dict and set, and what each operation costs; Names, objects and references

  6. Name each order’s customerMedium

    Python · from My import got 100 times slower when the data grew 10 times · Tests run here

    Topics: list, tuple, dict and set, and what each operation costs

  7. A repeat within k placesHard

    Python · from My import got 100 times slower when the data grew 10 times · Tests run here

    Topics: list, tuple, dict and set, and what each operation costs; Decisions and loops: if, for and while

  8. Flatten the pagesEasy

    Python · from I removed the bad readings, but one is still there · Tests run here

    Topics: Comprehensions

  9. Rows to columnsMedium

    Python · from I removed the bad readings, but one is still there · Tests run here

    Topics: Comprehensions; Mutable and immutable objects

  10. Keep the first of eachHard

    Python · from I removed the bad readings, but one is still there · Tests run here

    Topics: Comprehensions; list, tuple, dict and set, and what each operation costs

  11. A timer for a blockEasy

    Python · from After one error, debug mode stayed on for everyone · Tests run here

    Topics: Context managers

  12. Ignore some errorsMedium

    Python · from After one error, debug mode stayed on for everyone · Tests run here

    Topics: Context managers

  13. All or nothingHard

    Python · from After one error, debug mode stayed on for everyone · Tests run here

    Topics: Context managers; Exceptions

  14. FizzBuzzEasy

    Python · from My loop never stops · Tests run here

    Topics: Decisions and loops: if, for and while

  15. The first hot dayMedium

    Python · from My loop never stops · Tests run here

    Topics: Decisions and loops: if, for and while

  16. Guess the numberHard

    Python · from My loop never stops · Tests run here

    Topics: Decisions and loops: if, for and while

  17. Top error messagesEasy

    Python · from Count errors in a 10 GB log · Streaming · Tests run here

    Topics: Iterators and generators; collections, itertools and functools

  18. First errors from an endless logEasy

    Python · from Count errors in a 10 GB log · Streaming · Tests run here

    Topics: Iterators and generators

  19. Error spikes per minuteMedium

    Python · from Count errors in a 10 GB log · Streaming · Tests run here

    Topics: Iterators and generators; collections, itertools and functools

  20. The k slowest requestsHard

    Python · from Count errors in a 10 GB log · Top k · Tests run here

    Topics: Iterators and generators; Heaps: top-k, k-way merge, two heaps

  21. A point for a setEasy

    Python · from I added a field, and two different orders came out equal · Tests run here

    Topics: Dataclasses

  22. Sort version numbersMedium

    Python · from I added a field, and two different orders came out equal · Tests run here

    Topics: Dataclasses

  23. A line item that works out its totalHard

    Python · from I added a field, and two different orders came out equal · Tests run here

    Topics: Dataclasses

  24. Shout the resultEasy

    Python · from I pasted the same timing code into twenty functions · Tests run here

    Topics: Decorators, with arguments and functools.wraps

  25. Register pages with @routeMedium

    Python · from I pasted the same timing code into twenty functions · Tests run here

    Topics: Decorators, with arguments and functools.wraps

  26. Retry, with argumentsHard

    Python · from I pasted the same timing code into twenty functions · Tests run here

    Topics: Decorators, with arguments and functools.wraps; Exceptions

  27. Read the numbersEasy

    Python · from My server ignored the port I set · Tests run here

    Topics: Exceptions

  28. Try again after a timeoutMedium

    Python · from My server ignored the port I set · Tests run here

    Topics: Exceptions; Your first functions: def and return

  29. Wait longer each timeMedium

    Python · from My server ignored the port I set · Tests run here

    Topics: Exceptions; Your first functions: def and return

  30. All the orders, or noneHard

    Python · from My server ignored the port I set · Tests run here

    Topics: Exceptions; list, tuple, dict and set, and what each operation costs

  31. Save a reportEasy

    Python · from My script can’t find its file when I run it from another folder · Tests run here

    Topics: Files and pathlib

  32. Read the orders, skip the bad rowsEasy

    Python · from My script can’t find its file when I run it from another folder · Tests run here

    Topics: Files and pathlib; Exceptions

  33. Rotate the logsMedium

    Python · from My script can’t find its file when I run it from another folder · Tests run here

    Topics: Files and pathlib; Decisions and loops: if, for and while

  34. Find duplicate filesHard

    Python · from My script can’t find its file when I run it from another folder · Tests run here

    Topics: Files and pathlib; list, tuple, dict and set, and what each operation costs

  35. The averageEasy

    Python · from My function printed the total, but gave me None · Tests run here

    Topics: Your first functions: def and return

  36. Count the vowelsMedium

    Python · from My function printed the total, but gave me None · Tests run here

    Topics: Your first functions: def and return

  37. What’s wrong with the passwordHard

    Python · from My function printed the total, but gave me None · Tests run here

    Topics: Your first functions: def and return; Decisions and loops: if, for and while

  38. Hours and minutesEasy

    Python · from I closed the window and my code was gone · Tests run here

    Topics: Install Python and run your first file

  39. Split the billMedium

    Python · from I closed the window and my code was gone · Tests run here

    Topics: Install Python and run your first file

  40. Run it with numbersHard

    Python · from I closed the window and my code was gone · Tests run here

    Topics: Install Python and run your first file

  41. The largest, with a fallbackEasy

    Python · from My function remembers the last call · Tests run here

    Topics: Functions, *args, **kwargs and the mutable-default trap

  42. Build a link with a queryMedium

    Python · from My function remembers the last call · Tests run here

    Topics: Functions, *args, **kwargs and the mutable-default trap; Numbers, strings, slicing and f-strings

  43. Merge settings in layersHard

    Python · from My function remembers the last call · Tests run here

    Topics: Functions, *args, **kwargs and the mutable-default trap; Mutable and immutable objects

  44. Read git statusEasy

    Python · from I pushed my fix, and my teammate’s work disappeared · Tests run here

    Topics: Git for everyday work: commit, branch, merge, rebase, pull requests

  45. Merge one lineMedium

    Python · from I pushed my fix, and my teammate’s work disappeared · Tests run here

    Topics: Git for everyday work: commit, branch, merge, rebase, pull requests

  46. Replay my commitsHard

    Python · from I pushed my fix, and my teammate’s work disappeared · Tests run here

    Topics: Git for everyday work: commit, branch, merge, rebase, pull requests

  47. Split a formula into tokensEasy

    Python · from Python says my formula has too many nested parentheses · Tests run here

    Topics: How Python runs your code

  48. Turn a formula into stack stepsMedium

    Python · from Python says my formula has too many nested parentheses · Tests run here

    Topics: How Python runs your code

  49. Flatten a nested listMedium

    Python · from Python says my formula has too many nested parentheses · Tests run here

    Topics: How Python runs your code; Functions, *args, **kwargs and the mutable-default trap

  50. The order imports run inHard

    Python · from Python says my formula has too many nested parentheses · Tests run here

    Topics: How Python runs your code

  51. Missing or notEasy

    Python · from I started a new game, and the old board changed too · Tests run here

    Topics: is vs ==, shallow vs deep copy

  52. Settings that don’t leakMedium

    Python · from I started a new game, and the old board changed too · Tests run here

    Topics: is vs ==, shallow vs deep copy

  53. Find the shared rowsHard

    Python · from I started a new game, and the old board changed too · Tests run here

    Topics: is vs ==, shallow vs deep copy

  54. Quiet a chatty libraryEasy

    Python · from The bug happened at 3 a.m., and my prints told me nothing · Tests run here

    Topics: Logging

  55. Log the error properlyMedium

    Python · from The bug happened at 3 a.m., and my prints told me nothing · Tests run here

    Topics: Logging; Exceptions

  56. One JSON object per lineHard

    Python · from The bug happened at 3 a.m., and my prints told me nothing · Tests run here

    Topics: Logging

  57. From path to module nameEasy

    Python · from pip says it’s installed, but Python can’t find it · Tests run here

    Topics: Modules, packages and virtual environments

  58. Read a requirements fileMedium

    Python · from pip says it’s installed, but Python can’t find it · Tests run here

    Topics: Modules, packages and virtual environments

  59. Can two projects share one environment?Medium

    Python · from pip says it’s installed, but Python can’t find it · Tests run here

    Topics: Modules, packages and virtual environments; list, tuple, dict and set, and what each operation costs

  60. Count the distinct pointsEasy

    Python · from Python says my list can’t be a dict key · Tests run here

    Topics: Mutable and immutable objects; list, tuple, dict and set, and what each operation costs

  61. When does the playlist repeat?Medium

    Python · from Python says my list can’t be a dict key · Tests run here

    Topics: Mutable and immutable objects; Names, objects and references

  62. Remove duplicate recordsHard

    Python · from Python says my list can’t be a dict key · Tests run here

    Topics: Mutable and immutable objects; list, tuple, dict and set, and what each operation costs

  63. Make an empty gridEasy

    Python · from I changed my copy, and the original changed too · Tests run here

    Topics: Names, objects and references; list, tuple, dict and set, and what each operation costs

  64. Remove the negatives, in placeMedium

    Python · from I changed my copy, and the original changed too · Tests run here

    Topics: Names, objects and references; Your first functions: def and return

  65. Deep copy by handHard

    Python · from I changed my copy, and the original changed too · Tests run here

    Topics: Names, objects and references; list, tuple, dict and set, and what each operation costs

  66. Same backwardsEasy

    Python · from My shop’s total says 0.30000000000000004 · Tests run here

    Topics: Numbers, strings, slicing and f-strings

  67. Reverse the wordsEasy

    Python · from My shop’s total says 0.30000000000000004 · Tests run here

    Topics: Numbers, strings, slicing and f-strings

  68. The most common wordsMedium

    Python · from My shop’s total says 0.30000000000000004 · Tests run here

    Topics: Numbers, strings, slicing and f-strings; list, tuple, dict and set, and what each operation costs

  69. Add two numbers written as textHard

    Python · from My shop’s total says 0.30000000000000004 · Tests run here

    Topics: Numbers, strings, slicing and f-strings

  70. Money that adds upEasy

    Python · from My bank opened the same account twice · Tests run here

    Topics: Classes: inheritance, MRO, super() and dunder methods

  71. Shapes that know their areaMedium

    Python · from My bank opened the same account twice · Tests run here

    Topics: Classes: inheritance, MRO, super() and dunder methods

  72. A stack that works with len(), in and forHard

    Python · from My bank opened the same account twice · Tests run here

    Topics: Classes: inheritance, MRO, super() and dunder methods

  73. Average per group, skipping gapsEasy

    Python · from My sales report loop takes twenty minutes · Tests run here

    Topics: pandas basics: DataFrames, filtering, groupby and merge

  74. Join two tablesMedium

    Python · from My sales report loop takes twenty minutes · Tests run here

    Topics: pandas basics: DataFrames, filtering, groupby and merge

  75. A rolling averageHard

    Python · from My sales report loop takes twenty minutes · Tests run here

    Topics: pandas basics: DataFrames, filtering, groupby and merge

  76. Celsius and Fahrenheit togetherEasy

    Python · from Someone set the price to -5, and my report broke · Tests run here

    Topics: @property, class methods and static methods

  77. A total that’s always right and always fastMedium

    Python · from Someone set the price to -5, and my report broke · Tests run here

    Topics: @property, class methods and static methods

  78. Another way to make a dateHard

    Python · from Someone set the price to -5, and my report broke · Tests run here

    Topics: @property, class methods and static methods

  79. Leap yearsEasy

    Python · from I fixed one bug, and two old ones came back · Tests run here

    Topics: pytest basics

  80. Check the errorMedium

    Python · from I fixed one bug, and two old ones came back · Tests run here

    Topics: pytest basics; Exceptions

  81. Test it without sending emailHard

    Python · from I fixed one bug, and two old ones came back · Tests run here

    Topics: pytest basics

  82. The first check that failedEasy

    Python · from Python says my counter has no value · Tests run here

    Topics: Scope and the LEGB rule; Decisions and loops: if, for and while

  83. Clear the historyMedium

    Python · from Python says my counter has no value · Tests run here

    Topics: Scope and the LEGB rule; Mutable and immutable objects

  84. A scoreboardHard

    Python · from Python says my counter has no value · Tests run here

    Topics: Scope and the LEGB rule

  85. Group words by first letterEasy

    Python · from Counting words took me twenty lines, and it was slow · Tests run here

    Topics: collections, itertools and functools

  86. Unique, in orderMedium

    Python · from Counting words took me twenty lines, and it was slow · Tests run here

    Topics: collections, itertools and functools

  87. Chunks of a streamHard

    Python · from Counting words took me twenty lines, and it was slow · Tests run here

    Topics: collections, itertools and functools; Iterators and generators

  88. A hint that tells the truthEasy

    Python · from The total came out as “33”, three files away from the bug · Tests run here

    Topics: Type hints

  89. First item, any typeMedium

    Python · from The total came out as “33”, three files away from the bug · Tests run here

    Topics: Type hints

  90. Check inside a listHard

    Python · from The total came out as “33”, three files away from the bug · Tests run here

    Topics: Type hints

  91. Greet by nameEasy

    Python · from My calculator says 2 + 3 is 23 · Tests run here

    Topics: Variables, print() and input()

  92. Celsius to FahrenheitMedium

    Python · from My calculator says 2 + 3 is 23 · Tests run here

    Topics: Variables, print() and input()

  93. A receipt line that lines upHard

    Python · from My calculator says 2 + 3 is 23 · Tests run here

    Topics: Variables, print() and input()

  94. The cheapest productsEasy

    SQL · from The sales tile says two customers ordered, but it was only Hugo · Tests run here

    Topics: Aggregates

  95. Signed-in share of page viewsMedium

    SQL · from The sales tile says two customers ordered, but it was only Hugo · Tests run here

    Topics: Aggregates

  96. September in one rowMedium

    SQL · from The sales tile says two customers ordered, but it was only Hugo · Tests run here

    Topics: Aggregates

  97. When each product last soldEasy

    SQL · from Bigger than the order before, or just typed in late? · Tests run here

    Topics: Correlated subqueries; Joins: inner, left, right, full, self and cross

  98. Whose latest order was cancelled?Medium

    SQL · from Bigger than the order before, or just typed in late? · Tests run here

    Topics: Correlated subqueries; Simple subqueries

  99. Each customer’s biggest orderMedium

    SQL · from Bigger than the order before, or just typed in late? · Tests run here

    Topics: Correlated subqueries; Aggregates

  100. Each category’s share of salesEasy

    SQL · from A team of one made the above-average pay list · Tests run here

    Topics: CTEs; GROUP BY and HAVING

  101. The top 3 spendersMedium

    SQL · from A team of one made the above-average pay list · Tests run here

    Topics: CTEs; Simple subqueries

  102. Each department’s top earnerMedium

    SQL · from A team of one made the above-average pay list · Tests run here

    Topics: CTEs; Joins: inner, left, right, full, self and cross

  103. The city listEasy

    SQL · from Our “second-highest price” is the highest price · Tests run here

    Topics: DISTINCT

  104. Who put something in their cartEasy

    SQL · from Our “second-highest price” is the highest price · Tests run here

    Topics: DISTINCT

  105. The best-paid person in each departmentMedium

    SQL · from Our “second-highest price” is the highest price · Tests run here

    Topics: DISTINCT

  106. Ask headphone buyers for a reviewEasy

    SQL · from Grace got the window-shopper email twice · Tests run here

    Topics: EXISTS vs IN vs JOIN, and the NOT IN + NULL trap; DISTINCT

  107. Customers who are all paid upMedium

    SQL · from Grace got the window-shopper email twice · Tests run here

    Topics: EXISTS vs IN vs JOIN, and the NOT IN + NULL trap; Correlated subqueries

  108. What wallet customers spendMedium

    SQL · from Grace got the window-shopper email twice · Tests run here

    Topics: EXISTS vs IN vs JOIN, and the NOT IN + NULL trap; GROUP BY and HAVING

  109. The most active customersEasy

    SQL · from Grace shopped on one day and got the loyalty coupon · Tests run here

    Topics: GROUP BY and HAVING

  110. Revenue for each month of 2026Medium

    SQL · from Grace shopped on one day and got the loyalty coupon · Tests run here

    Topics: GROUP BY and HAVING

  111. Accounts that share an emailMedium

    SQL · from Grace shopped on one day and got the loyalty coupon · Tests run here

    Topics: GROUP BY and HAVING

  112. Headcount for the staff pageEasy

    SQL · from Dana never paid, and finance never noticed · Tests run here

    Topics: Joins: inner, left, right, full, self and cross; GROUP BY and HAVING

  113. Products nobody has boughtMedium

    SQL · from Dana never paid, and finance never noticed · Tests run here

    Topics: Joins: inner, left, right, full, self and cross

  114. The raise budgetMedium

    SQL · from Dana never paid, and finance never noticed · Tests run here

    Topics: Joins: inner, left, right, full, self and cross

  115. Each customer's two latest ordersEasy

    SQL · from Each customer's latest order · Top N per group · Tests run here

    Topics: Window functions: ranking, LAG, LEAD, running totals, frames

  116. The most expensive product in each category, ties includedMedium

    SQL · from Each customer's latest order · Top N per group · Tests run here

    Topics: Window functions: ranking, LAG, LEAD, running totals, frames

  117. Days since each customer's previous orderMedium

    SQL · from Each customer's latest order · Previous row · Tests run here

    Topics: Window functions: ranking, LAG, LEAD, running totals, frames; Date and string functions

  118. Running total per customerHard

    SQL · from Each customer's latest order · Running total · Tests run here

    Topics: Window functions: ranking, LAG, LEAD, running totals, frames

  119. Free delivery after the discountEasy

    SQL · from Dana first ordered in August and got September’s welcome email · Tests run here

    Topics: Logical query processing order

  120. The two newest customersMedium

    SQL · from Dana first ordered in August and got September’s welcome email · Tests run here

    Topics: Logical query processing order; Joins: inner, left, right, full, self and cross

  121. Customers per cityMedium

    SQL · from Dana first ordered in August and got September’s welcome email · Tests run here

    Topics: Logical query processing order; GROUP BY and HAVING

  122. The supplier’s price importEasy

    SQL · from We deleted Ben’s duplicate payment, and the real one went too · Tests run here

    Topics: INSERT, UPDATE and DELETE

  123. Gift card balancesMedium

    SQL · from We deleted Ben’s duplicate payment, and the real one went too · Tests run here

    Topics: Data types and constraints

  124. Dana closes her accountMedium

    SQL · from We deleted Ben’s duplicate payment, and the real one went too · Tests run here

    Topics: INSERT, UPDATE and DELETE; Tables, rows and keys

  125. Who still needs a managerEasy

    SQL · from A guest checked out, and nobody got a reminder · Tests run here

    Topics: NULL and three-valued logic

  126. The Sales offsiteMedium

    SQL · from A guest checked out, and nobody got a reminder · Tests run here

    Topics: NULL and three-valued logic

  127. Left in the cartMedium

    SQL · from A guest checked out, and nobody got a reminder · Tests run here

    Topics: NULL and three-valued logic; GROUP BY and HAVING

  128. Cheap stationery and home productsEasy

    SQL · from Order 109 disappeared from the warehouse’s list · Tests run here

    Topics: SELECT, WHERE, ORDER BY and LIMIT

  129. The next page of customersMedium

    SQL · from Order 109 disappeared from the warehouse’s list · Tests run here

    Topics: SELECT, WHERE, ORDER BY and LIMIT

  130. Customers with an example.com emailMedium

    SQL · from Order 109 disappeared from the warehouse’s list · Tests run here

    Topics: SELECT, WHERE, ORDER BY and LIMIT

  131. Paid as much as their managerEasy

    SQL · from Grace cancelled, and we still asked if she’d ordered twice · Tests run here

    Topics: Joins: inner, left, right, full, self and cross

  132. Onboarding buddiesMedium

    SQL · from Grace cancelled, and we still asked if she’d ordered twice · Tests run here

    Topics: Joins: inner, left, right, full, self and cross

  133. Checkouts against ordersMedium

    SQL · from Grace cancelled, and we still asked if she’d ordered twice · Tests run here

    Topics: Joins: inner, left, right, full, self and cross; GROUP BY and HAVING

  134. Orders to LisbonEasy

    SQL · from Elif bought two backpacks and got the bottle offer twice · Tests run here

    Topics: Simple subqueries

  135. The top category each monthMedium

    SQL · from Elif bought two backpacks and got the bottle offer twice · Tests run here

    Topics: Simple subqueries; GROUP BY and HAVING

  136. The busiest dayMedium

    SQL · from Elif bought two backpacks and got the bottle offer twice · Tests run here

    Topics: Simple subqueries; GROUP BY and HAVING

  137. One wishlist row per productEasy

    SQL · from My report shows reviews of a product we don’t sell · Tests run here

    Topics: Tables, rows and keys

  138. Categories inside categoriesMedium

    SQL · from My report shows reviews of a product we don’t sell · Tests run here

    Topics: Tables, rows and keys

  139. Addresses go when the customer goesMedium

    SQL · from My report shows reviews of a product we don’t sell · Tests run here

    Topics: Tables, rows and keys

  140. Move the zeros to the endEasy

    DSA · from I reversed the list, but the caller’s list didn’t change · Tests run here

    Topics: Arrays and strings

  141. A palindrome, skipping spaces and caseEasy

    DSA · from I reversed the list, but the caller’s list didn’t change · Two pointers · Tests run here

    Topics: Arrays and strings; Two pointers

  142. Rotate a listMedium

    DSA · from I reversed the list, but the caller’s list didn’t change · Tests run here

    Topics: Arrays and strings

  143. Sort orders by dayEasy

    DSA · from My sort is right, but 50,000 scores take half a minute · Sorting · Tests run here

    Topics: Basic sorting

  144. Ids in both listsEasy

    DSA · from My sort is right, but 50,000 scores take half a minute · Two pointers · Tests run here

    Topics: Basic sorting; Two pointers; Hashing and hash maps

  145. H-indexMedium

    DSA · from My sort is right, but 50,000 scores take half a minute · Sorting · Tests run here

    Topics: Basic sorting

  146. The first repeated idEasy

    DSA · from My code passed every test, then timed out on the big one · Tests run here

    Topics: Big-O for time and space; Hashing and hash maps

  147. The missing ticketMedium

    DSA · from My code passed every test, then timed out on the big one · Tests run here

    Topics: Big-O for time and space

  148. The best day to buyMedium

    DSA · from My code passed every test, then timed out on the big one · Tests run here

    Topics: Big-O for time and space; Arrays and strings

  149. The first bad buildEasy

    DSA · from My binary search says 0 isn’t in [4, 5, 6, 7, 0, 1, 2] · Binary search · Tests run here

    Topics: Binary search on an index and on the answer

  150. The smallest vanMedium

    DSA · from My binary search says 0 isn’t in [4, 5, 6, 7, 0, 1, 2] · Binary search · Tests run here

    Topics: Binary search on an index and on the answer

  151. A reading planHard

    DSA · from My binary search says 0 isn’t in [4, 5, 6, 7, 0, 1, 2] · Binary search · Tests run here

    Topics: Binary search on an index and on the answer

  152. Cut a note from a magazineEasy

    DSA · from My anagram check says aab and abb match · Hash map · Tests run here

    Topics: Hashing and hash maps

  153. Words that follow a patternEasy

    DSA · from My anagram check says aab and abb match · Hash map · Tests run here

    Topics: Hashing and hash maps

  154. The longest run of numbers in a rowMedium

    DSA · from My anagram check says aab and abb match · Hash map · Tests run here

    Topics: Hashing and hash maps; Basic sorting

  155. Buying tickets in a lineEasy

    DSA · from My rate limiter let 200 requests through in one second · Queue · Tests run here

    Topics: Stacks and queues

  156. A queue made of two stacksEasy

    DSA · from My rate limiter let 200 requests through in one second · Queue · Tests run here

    Topics: Stacks and queues; Designing data structures: LRU and LFU caches, min stack, randomized set

  157. The first user who sent just one messageMedium

    DSA · from My rate limiter let 200 requests through in one second · Queue · Tests run here

    Topics: Stacks and queues; Hashing and hash maps

  158. The size of a folderEasy

    DSA · from My power function crashes when n is 5,000 · Recursion · Tests run here

    Topics: Recursion

  159. Decode a stringMedium

    DSA · from My power function crashes when n is 5,000 · Recursion · Tests run here

    Topics: Recursion; Stacks and queues

  160. The k-th symbol in a rowMedium

    DSA · from My power function crashes when n is 5,000 · Recursion · Tests run here

    Topics: Recursion

  161. The biggest sum of k in a rowEasy

    DSA · from My no-repeat finder says 2, but “vdf” has 3 · Sliding window · Tests run here

    Topics: Sliding window

  162. The fewest days to reach a goalMedium

    DSA · from My no-repeat finder says 2, but “vdf” has 3 · Sliding window · Tests run here

    Topics: Sliding window

  163. The shortest stretch that holds a wordHard

    DSA · from My no-repeat finder says 2, but “vdf” has 3 · Sliding window · Tests run here

    Topics: Sliding window; Hashing and hash maps

  164. Typing with backspaceEasy

    DSA · from My bracket check says ([)] is fine · Stack · Tests run here

    Topics: Stacks and queues; Two pointers

  165. Daily temperaturesMedium

    DSA · from My bracket check says ([)] is fine · Monotonic stack · Tests run here

    Topics: Stacks and queues; Monotonic stack and queue

  166. Reverse Polish notationMedium

    DSA · from My bracket check says ([)] is fine · Stack · Tests run here

    Topics: Stacks and queues

  167. A palindrome, if you delete one letterEasy

    DSA · from My tank finder says 40, but two walls hold 49 · Two pointers · Tests run here

    Topics: Two pointers; Arrays and strings

  168. The fewest boxes, two items eachMedium

    DSA · from My tank finder says 40, but two walls hold 49 · Two pointers · Tests run here

    Topics: Two pointers; Basic sorting

  169. Rain on a row of barsHard

    DSA · from My tank finder says 40, but two walls hold 49 · Two pointers · Tests run here

    Topics: Two pointers

  170. Pair sum in a sorted listEasy

    DSA · from Two Sum · Two pointers · Tests run here

    Topics: Two pointers

  171. Count pairs with a given differenceMedium

    DSA · from Two Sum · Hash map · Tests run here

    Topics: Hashing and hash maps

  172. Three SumMedium

    DSA · from Two Sum · Two pointers · Tests run here

    Topics: Two pointers; Basic sorting

  173. Four SumHard

    DSA · from Two Sum · Two pointers · Tests run here

    Topics: Two pointers; Basic sorting

  174. Endpoints for a libraryEasy

    System Design · from The phone app makes five calls to draw one screen · Think it through, then compare answers

    Topics: REST vs GraphQL vs gRPC

  175. An API for partnersMedium

    System Design · from The phone app makes five calls to draw one screen · Think it through, then compare answers

    Topics: REST vs GraphQL vs gRPC

  176. Live prices on 10,000 screensHard

    System Design · from The phone app makes five calls to draw one screen · Think it through, then compare answers

    Topics: REST vs GraphQL vs gRPC

  177. What to cache?Easy

    System Design · from I added a cache, and now the shop shows the old price · Think it through, then compare answers

    Topics: Caching: cache-aside, write-through, write-back, eviction, TTL

  178. A home page for everyoneMedium

    System Design · from I added a cache, and now the shop shows the old price · Think it through, then compare answers

    Topics: Caching: cache-aside, write-through, write-back, eviction, TTL; CDNs

  179. A likes counterHard

    System Design · from I added a cache, and now the shop shows the old price · Think it through, then compare answers

    Topics: Caching: cache-aside, write-through, write-back, eviction, TTL

  180. Cache headersEasy

    System Design · from Users in Sydney wait three seconds for images from Virginia · Think it through, then compare answers

    Topics: CDNs

  181. Private filesMedium

    System Design · from Users in Sydney wait three seconds for images from Virginia · Think it through, then compare answers

    Topics: CDNs; Object storage

  182. Video for millionsHard

    System Design · from Users in Sydney wait three seconds for images from Virginia · Think it through, then compare answers

    Topics: CDNs

  183. Status codes for an APIEasy

    System Design · from My page takes two seconds, and I can’t say where the time goes · Think it through, then compare answers

    Topics: Client–server, HTTP and HTTPS

  184. Slow on the other side of the worldMedium

    System Design · from My page takes two seconds, and I can’t say where the time goes · Think it through, then compare answers

    Topics: Client–server, HTTP and HTTPS; Latency numbers worth knowing

  185. A login form over HTTPMedium

    System Design · from My page takes two seconds, and I can’t say where the time goes · Think it through, then compare answers

    Topics: Client–server, HTTP and HTTPS

  186. Which record?Easy

    System Design · from I moved my site, but some users still reach the old server · Think it through, then compare answers

    Topics: DNS

  187. Two regions, one nameMedium

    System Design · from I moved my site, but some users still reach the old server · Think it through, then compare answers

    Topics: DNS; Load balancers: L4 vs L7 and algorithms

  188. Your DNS provider goes downHard

    System Design · from I moved my site, but some users still reach the old server · Think it through, then compare answers

    Topics: DNS

  189. Chat app numbersEasy

    System Design · from The interviewer asks how much storage a photo app needs, and I freeze · Think it through, then compare answers

    Topics: Back-of-the-envelope estimates

  190. Logs per dayMedium

    System Design · from The interviewer asks how much storage a photo app needs, and I freeze · Think it through, then compare answers

    Topics: Back-of-the-envelope estimates

  191. Video storageHard

    System Design · from The interviewer asks how much storage a photo app needs, and I freeze · Think it through, then compare answers

    Topics: Back-of-the-envelope estimates

  192. Fastest to slowestEasy

    System Design · from My page makes 50 database calls and takes four seconds · Think it through, then compare answers

    Topics: Latency numbers worth knowing

  193. Read 10 GBMedium

    System Design · from My page makes 50 database calls and takes four seconds · Think it through, then compare answers

    Topics: Latency numbers worth knowing

  194. Six calls in checkoutHard

    System Design · from My page makes 50 database calls and takes four seconds · Think it through, then compare answers

    Topics: Latency numbers worth knowing

  195. A chat app with WebSocketsEasy

    System Design · from One server is at 95% while the other two sit idle · Think it through, then compare answers

    Topics: Load balancers: L4 vs L7 and algorithms

  196. A new version to 5% of usersMedium

    System Design · from One server is at 95% while the other two sit idle · Think it through, then compare answers

    Topics: Load balancers: L4 vs L7 and algorithms

  197. Cache servers by keyHard

    System Design · from One server is at 95% while the other two sit idle · Think it through, then compare answers

    Topics: Load balancers: L4 vs L7 and algorithms; Partitioning and consistent hashing

  198. The nightly reportEasy

    System Design · from On sale day, our one server hit 100% and the shop went down · Think it through, then compare answers

    Topics: Vertical vs horizontal scaling

  199. Uploads on one serverMedium

    System Design · from On sale day, our one server hit 100% and the shop went down · Think it through, then compare answers

    Topics: Vertical vs horizontal scaling; Object storage

  200. Now the database is at 100%Hard

    System Design · from On sale day, our one server hit 100% and the shop went down · Think it through, then compare answers

    Topics: Vertical vs horizontal scaling; Replication

  201. Pick a storeEasy

    System Design · from Our orders are fine, but the activity feed has two billion rows · Think it through, then compare answers

    Topics: SQL vs NoSQL

  202. Product attributes that varyMedium

    System Design · from Our orders are fine, but the activity feed has two billion rows · Think it through, then compare answers

    Topics: SQL vs NoSQL

  203. Chat messagesHard

    System Design · from Our orders are fine, but the activity feed has two billion rows · Think it through, then compare answers

    Topics: SQL vs NoSQL; Partitioning and consistent hashing

  204. A pastebinEasy

    System Design · from Design a URL shortener · Think it through, then compare answers

    Topics: Case studies: URL shortener, rate limiter, chat, news feed, video, ride-hailing, payments and more; Object storage

  205. Click analyticsMedium

    System Design · from Design a URL shortener · Think it through, then compare answers

    Topics: Queues and pub/sub; Batch vs stream processing

  206. Rate-limit link creationMedium

    System Design · from Design a URL shortener · Think it through, then compare answers

    Topics: Rate-limiting algorithms

  207. Custom aliases, worldwideHard

    System Design · from Design a URL shortener · Think it through, then compare answers

    Topics: Multi-region and disaster recovery; Unique ID generation

  208. Does it need a preflight?Easy

    Frameworks · from The browser blocked my front end from calling my own API · Tests run here

    Topics: CORS and CSRF

  209. Check the CSRF tokenMedium

    Frameworks · from The browser blocked my front end from calling my own API · Tests run here

    Topics: CORS and CSRF

  210. Is this origin ours?Medium

    Frameworks · from The browser blocked my front end from calling my own API · Tests run here

    Topics: CORS and CSRF

  211. The pluralize filterEasy

    Frameworks · from The note form showed one mistake at a time, and saved spaces as a title · Tests run here

    Topics: Django: apps, MTV, views, the ORM, migrations, admin, forms, middleware, auth

  212. Django-style lookupsMedium

    Frameworks · from The note form showed one mistake at a time, and saved spaces as a title · Tests run here

    Topics: Django: apps, MTV, views, the ORM, migrations, admin, forms, middleware, auth

  213. Django’s get_pageMedium

    Frameworks · from The note form showed one mistake at a time, and saved spaces as a title · Tests run here

    Topics: Django: apps, MTV, views, the ORM, migrations, admin, forms, middleware, auth

  214. Reviews with their authorsEasy

    Frameworks · from The order list page is slow · N+1 queries · Think it through, then compare answers

    Topics: Django ORM performance: N+1, select_related, prefetch_related

  215. Blog posts with their tagsMedium

    Frameworks · from The order list page is slow · N+1 queries · Think it through, then compare answers

    Topics: Django ORM performance: N+1, select_related, prefetch_related

  216. Customers with only their shipped ordersMedium

    Frameworks · from The order list page is slow · N+1 queries · Think it through, then compare answers

    Topics: Django ORM performance: N+1, select_related, prefetch_related

  217. An API that counts in a propertyHard

    Frameworks · from The order list page is slow · N+1 queries · Think it through, then compare answers

    Topics: Django ORM performance: N+1, select_related, prefetch_related; Django REST Framework: serializers, viewsets, permissions, throttling, pagination

  218. Owner, or read onlyEasy

    Frameworks · from A user made someone else the owner of a note by adding one field to the JSON · Tests run here

    Topics: Django REST Framework: serializers, viewsets, permissions, throttling, pagination

  219. A rate throttleMedium

    Frameworks · from A user made someone else the owner of a note by adding one field to the JSON · Tests run here

    Topics: Django REST Framework: serializers, viewsets, permissions, throttling, pagination

  220. Next and previous linksMedium

    Frameworks · from A user made someone else the owner of a note by adding one field to the JSON · Tests run here

    Topics: Django REST Framework: serializers, viewsets, permissions, throttling, pagination

  221. Typed path parametersMedium

    Frameworks · from Every request opened two database sessions, and closed none · Tests run here

    Topics: FastAPI: parameters, Pydantic v2, response models, dependency injection, routers

  222. A response modelEasy

    Frameworks · from Every request opened two database sessions, and closed none · Tests run here

    Topics: FastAPI: parameters, Pydantic v2, response models, dependency injection, routers

  223. Query parameters with every errorMedium

    Frameworks · from Every request opened two database sessions, and closed none · Tests run here

    Topics: FastAPI: parameters, Pydantic v2, response models, dependency injection, routers

  224. A login_required decoratorEasy

    Frameworks · from My Flask tests pass one at a time, and fail together · Tests run here

    Topics: Flask: routing, contexts, blueprints, app factory, config, Jinja2, SQLAlchemy, testing

  225. Build a URL from its endpointMedium

    Frameworks · from My Flask tests pass one at a time, and fail together · Tests run here

    Topics: Flask: routing, contexts, blueprints, app factory, config, Jinja2, SQLAlchemy, testing

  226. A database session per requestMedium

    Frameworks · from My Flask tests pass one at a time, and fail together · Tests run here

    Topics: Flask: routing, contexts, blueprints, app factory, config, Jinja2, SQLAlchemy, testing

  227. Read the status lineEasy

    Frameworks · from My app said “Saved” when nothing was saved · Tests run here

    Topics: HTTP methods and status codes

  228. Is it safe to try again?Medium

    Frameworks · from My app said “Saved” when nothing was saved · Tests run here

    Topics: HTTP methods and status codes

  229. Read a raw requestMedium

    Frameworks · from My app said “Saved” when nothing was saved · Tests run here

    Topics: HTTP methods and status codes

  230. Which migrations are left?Easy

    Frameworks · from The second deploy crashed: table notes already exists · Tests run here

    Topics: ORMs and migrations

  231. Rows into objectsMedium

    Frameworks · from The second deploy crashed: table notes already exists · Tests run here

    Topics: ORMs and migrations

  232. Flag a risky migrationMedium

    Frameworks · from The second deploy crashed: table notes already exists · Tests run here

    Topics: ORMs and migrations

  233. Show a comment safelyEasy

    Frameworks · from Someone typed a quote into our search box and read other users’ notes · Tests run here

    Topics: Top OWASP risks: injection, XSS, broken authentication

  234. Slow down password guessingMedium

    Frameworks · from Someone typed a quote into our search box and read other users’ notes · Tests run here

    Topics: Top OWASP risks: injection, XSS, broken authentication

  235. Who may edit this note?Medium

    Frameworks · from Someone typed a quote into our search box and read other users’ notes · Tests run here

    Topics: Top OWASP risks: injection, XSS, broken authentication

  236. Match a routeEasy

    Frameworks · from Our error pages lost their security headers · Tests run here

    Topics: The request lifecycle

  237. Give every request an idMedium

    Frameworks · from Our error pages lost their security headers · Tests run here

    Topics: The request lifecycle

  238. Time every requestMedium

    Frameworks · from Our error pages lost their security headers · Tests run here

    Topics: The request lifecycle

  239. Build the next linkEasy

    Frameworks · from The same note showed up twice as I scrolled · Tests run here

    Topics: REST design

  240. Filter and sort from the queryMedium

    Frameworks · from The same note showed up twice as I scrolled · Tests run here

    Topics: REST design

  241. Change some fields with PATCHMedium

    Frameworks · from The same note showed up twice as I scrolled · Tests run here

    Topics: REST design

  242. A safe session cookieEasy

    Frameworks · from Users are logged out at random behind the load balancer · Tests run here

    Topics: Sessions, JWT and OAuth2

  243. Refresh tokens that work onceMedium

    Frameworks · from Users are logged out at random behind the load balancer · Tests run here

    Topics: Sessions, JWT and OAuth2

  244. The PKCE challengeMedium

    Frameworks · from Users are logged out at random behind the load balancer · Tests run here

    Topics: Sessions, JWT and OAuth2

  245. Read a query stringEasy

    Frameworks · from 200 open chats froze my Flask server · Tests run here

    Topics: WSGI vs ASGI

  246. Middleware that adds a headerMedium

    Frameworks · from 200 open chats froze my Flask server · Tests run here

    Topics: WSGI vs ASGI

  247. How many workers?Medium

    Frameworks · from 200 open chats froze my Flask server · Tests run here

    Topics: WSGI vs ASGI

  248. Split long docs into chunksEasy

    AI Engineering · from The chatbot doesn’t know your docs · RAG · Think it through, then compare answers

    Topics: RAG basics: why, chunking, embedding, top-k

  249. Merge two result listsMedium

    AI Engineering · from The chatbot doesn’t know your docs · RAG · Think it through, then compare answers

    Topics: Advanced RAG: hybrid search, reranking, query rewriting, metadata filters, GraphRAG, agentic RAG; Vector databases and ANN indexes: HNSW, IVF, PQ

  250. The bot answers confidently and wronglyMedium

    AI Engineering · from The chatbot doesn’t know your docs · RAG · Think it through, then compare answers

    Topics: Why models hallucinate; RAG basics: why, chunking, embedding, top-k

  251. Only show docs the user may seeHard

    AI Engineering · from The chatbot doesn’t know your docs · RAG · Think it through, then compare answers

    Topics: Advanced RAG: hybrid search, reranking, query rewriting, metadata filters, GraphRAG, agentic RAG; Safety: prompt injection, jailbreaks, data leakage and PII

  252. The error rate jumpedEasy

    Scenarios · from p95 latency tripled after a deploy · Think it through, then compare answers

    Topics: The error rate jumped

  253. Database CPU at 100% every morningMedium

    Scenarios · from p95 latency tripled after a deploy · Think it through, then compare answers

    Topics: Production: memory leak, DB CPU at 100%, deadlocks, disk full, intermittent 502s; A 40-second report, duplicate rows, dashboards that disagree

  254. Intermittent 502sMedium

    Scenarios · from p95 latency tripled after a deploy · Think it through, then compare answers

    Topics: Production: memory leak, DB CPU at 100%, deadlocks, disk full, intermittent 502s

  255. A cache stampede on the hourHard

    Scenarios · from p95 latency tripled after a deploy · Think it through, then compare answers

    Topics: Cache stampede and queue backlog

  256. Clarifying questionsEasy

    Interview Playbook · from The coding round, step by step · Think it through, then compare answers

    Topics: The coding-round method: clarify, examples, brute force, optimize, code, test, analyze

  257. Brute force to betterEasy

    Interview Playbook · from The coding round, step by step · Think it through, then compare answers

    Topics: The coding-round method: clarify, examples, brute force, optimize, code, test, analyze

  258. Test your own codeMedium

    Interview Playbook · from The coding round, step by step · Think it through, then compare answers

    Topics: The coding-round method: clarify, examples, brute force, optimize, code, test, analyze

  259. Ten minutes leftHard

    Interview Playbook · from The coding round, step by step · Think it through, then compare answers

    Topics: The coding-round method: clarify, examples, brute force, optimize, code, test, analyze

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