FAANG · LeetCode #525

Contiguous Array — three-stage drill

1The ProblemMedium

Contiguous Array

Longest contiguous subarray with an equal number of 0s and 1s.

Examples

[0,1,0,0,1,1,0]→ 6[1,1,1]→ 0

constraintOnly 0s and 1s.

2The PatternTIME O(n) · SPACE O(n)

Prefix Sum

Running cumulative totals turn range-sum questions into O(1) lookups.

Problems using this pattern

3The SolutionRESULT → 6

Step through it

start: seen = {0: -1} (balance 0 before the array), best = 0

Step 1 / 9

Python▎balance + first-index map

1def findMaxLength(nums):
2    seen = {0: -1}
3    balance = best = 0
4    for i, x in enumerate(nums):
5        balance += 1 if x == 1 else -1
6        if balance in seen:
7            best = max(best, i - seen[balance])
8        else:
9            seen[balance] = i
10    return best

why it worksMap 0 → −1 so 'equal 0s and 1s' becomes 'running sum 0'. Store the FIRST index each balance was seen (not a count): if the same balance reappears at index i, everything between cancels out, so the subarray (firstIdx, i] is balanced — length i − firstIdx. Keep the longest.

Arrow keys to navigate · escape to reset

All problems by pattern

Two Pointers

Sliding Window

Prefix Sum

Monotonic Deque

Shortest Subarray with Sum at Least K

Binary Search

Arrays & Hashing

Monotonic Stack

Intervals

Contiguous Array — three-stage drill