CodeForces 873B Balanced Substring (预处理)

把0当成-1预处理所有前缀和即可

#include<bits/stdc++.h>
using namespace std;

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))
#define pii pair<int,int>
#define fi first
#define se second
#define mk(x,y) make_pair(x,y)
const int mod=1e9+7;
const int maxn=1e5+100;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){
    if(y==0) return x;
    return gcd(y,x%y);
}
int n;
char s[maxn];
vector<int> g[maxn<<1];
int main(){
    cin>>n;
    int v=0,ans=0;
    g[maxn].push_back(0);
    scanf("%s",s);
    rep(i,0,n){
        if(s[i]=='0') v--;
        else v++;
        g[v+maxn].push_back(i+1);
    }
    rep(i,0,maxn<<1){
        int sz=g[i].size();
        if(sz==0) continue;
        sort(g[i].begin(),g[i].end());
        ans=max(ans,g[i][sz-1]-g[i][0]);
    }
    cout<<ans;
    return 0;
}