Longest Substring with Equal Number of 0s and 1s

Problem Given a binary string consisting only of the characters '0' and '1', find the length of the longest contiguous substring that contains an equal number of 0s and 1s.

Input / Output

• Input: a string s over the alphabet {'0','1'}.
• Output: an integer — the length of the longest balanced substring, or 0 if no such substring exists.

Constraints

• 1 <= |s| <= 10^5
• Characters are limited to '0' and '1'.
• Enumerating all substrings is O(n^2) and too slow at this size.

Example

• s = "11010011" → 8; the whole string holds four 0s and four 1s.
• s = "1100011" → 6; the prefix "110001" balances at three of each, while the full string does not.
• s = "0000" → 0; no balanced substring exists.