You are given a string s consisting only of characters 0 and 1. A substring [l, r] of s is a string s_ls_l + 1s_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.

The first line contains n (1 ≤ n ≤ 100000) — the number of characters in s.

The second line contains a string s consisting of exactly n characters. Only characters 0 and 1 can appear in s

If there is no non-empty balanced substring in s, print 0. Otherwise, print the length of the longest balanced substring.

8
11010111

4

3
111

0

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.

求前綴和，然後map一下，前綴和相同的表示兩位置之間的字符串達到了平衡。

代碼：

#include <iostream>
#include <string>
#include <map>
using namespace std;
int s[100001];
int main()
{
    int n;
    string a;
    cin>>n;
    cin>>a;
    map<int,int>q;
    int sum = 0,maxi = 0;
    for(int i = 0;i < n;i ++)
    {
        if(a[i] == ‘0‘)sum += -1;
        else sum += 1;
        if(!q[sum]&&sum)q[sum] = i + 1;//如果sum為0表示本來就平衡不需要標記了
        else
        {
            maxi = max(maxi,i - q[sum] + 1);
        }
    }
    cout<<maxi;
}

Balanced Substring