51nod1393

思路：一个位num0-num1值=某位num0-num1值相等就代表这段区间内01数字相等，然后还要判断当前位置num0==num1这个情况

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

const int N=1e6+10;

char s[N];
int sumone[N];
int sumzero[N];
int mp[2*N];

int main()
{
    int len,x;
    sumone[0]=0;
    sumzero[0]=0;
    scanf("%s",s+1);
    len=strlen(s+1);
    int ans=0;
    memset(mp,-1,sizeof(mp));
    for(int i=1;i<=len;i++)
    {
        if((s[i]-'0')==0)
        {
            sumzero[i]=sumzero[i-1]+1;
            sumone[i]=sumone[i-1];
            x=sumone[i]-sumzero[i]+500000;
            if(mp[x]==-1)
                mp[x]=i;
            else
                ans=max(ans,i-mp[x]);
            if(sumone[i]==sumzero[i])
                ans=max(ans,i);
        }
        else
        {
            sumone[i]=sumone[i-1]+1;
            sumzero[i]=sumzero[i-1];
            x=sumone[i]-sumzero[i]+500000;
            if(mp[x]==-1)
                mp[x]=i;
            else
                ans=max(ans,i-mp[x]);
            if(sumone[i]==sumzero[i])
                ans=max(ans,i);
        }
    }
    printf("%d\n",ans);
    return 0;
}