CF873B Balanced Substring(前缀和)

#include<iostream>
#include<cstring>
#include<cstdio>
#include<map>
#include<algorithm>
#define ull unsigned long long
using namespace std;
typedef long long ll;
typedef pair<int,int> pll;
const int N=1e5+10;
int sum;
map<int,int> m1;
int main(){
    int n;
    cin>>n;
    string s;
    cin>>s;
    int i;
    s=" "+s;
    m1[0]=0;
    int ans=0;
    for(i=1;i<=n;i++){
        if(s[i]==‘0‘)
            sum--;
        else
            sum++;
        if(m1.count(sum))
            ans=max(ans,i-m1[sum]);
        else
            m1[sum]=i;
    }
    cout<<ans<<endl;


}