PrepArena

Endless mode

Fix bug after bug. Keep the streak alive.

Every level is a correct algorithm with one injected bug. All 147 bugs were verified by actually running the tests — no fake levels, no bugs that secretly still pass. Level numbers are stable, so you can share a link to a tricky one.

Start level 1

Current streak

0

solves in a row without revealing the fix

Best streak

0

+5 XP bonus per streak step (max +50)

Bugs fixed

0 / 147

7 kinds of bugs

  • Off-by-one — Something is off by exactly one: a + 1 / - 1, or a ++ / -- going the wrong way.
  • Comparison — A comparison operator is wrong: think < vs <=, > vs >=, or the direction of the check.
  • Logic — A boolean check is inverted or uses the wrong connective (=== vs !==, && vs ||).
  • Initial value — A variable starts with the wrong value.
  • Arithmetic — An update adds where it should subtract (or the other way round).
  • Wrong function — A built-in function is swapped for its opposite.
  • Return value — The function returns the wrong boolean somewhere.

20 algorithms

  • Sum Zero · Two pointers
  • Valid Anagram · Frequency counter
  • Count Unique Values · Two pointers
  • Max Sum of k Consecutive · Sliding window
  • Binary Search · Divide & conquer
  • Squares in Another Array · Frequency counter
  • Factorial · Recursion
  • Power · Recursion
  • Fibonacci (memoised) · Recursion
  • Bubble Sort (optimised) · Sorting
  • Selection Sort · Sorting
  • Insertion Sort · Sorting
  • Maximum Occurring Character · Frequency counter
  • Palindrome Check · Two pointers
  • Longest Substring Without Repeats · Sliding window
  • Two Sum · Hash map
  • Best Time to Buy and Sell Stock · Greedy
  • Valid Parentheses · Stack
  • Move Zeroes · Two pointers
  • Merge Two Sorted Arrays · Two pointers