CF873B Balanced Substring

You are given a string $ s $ consisting only of characters 0 and 1. A substring $ [l,r] $ of $ s $ is a string $ s_{l}s_{l+1}s_{l+2}...\ s_{r} $ , and its length equals to $ r-l+1 $ . A substring is called balanced if the number of zeroes (0) equals to the number of ones in this substring. You have to determine the length of the longest balanced substring of $ s $ .

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In the first example you can choose the substring $ [3,6] $ . It is balanced, and its length is $ 4 $ . Choosing the substring $ [2,5] $ is also possible. In the second example it's impossible to find a non-empty balanced substring.