2019牛客多校第三场B-Crazy Binary String(前缀和+思维) Crazy Binary String

题目传送门

解题思路

把1记为1，把0记为-1，然后求前缀和，前缀和相等的就说明中间的01数一样。只要记录前缀和数值出现的位置即可更新出答案。

代码如下

#include <bits/stdc++.h>
#define INF 0x3f3f3f3f
using namespace std;
typedef long long ll;

const int N = 100005;

int l[N << 1];
int sum[N];

int main()
{
    int n;
    scanf("%d%*c", &n);
    int ans1 = 0;
    int a, b;
    a = b = 0;
    for(int i = 1; i <= n; i ++){
        char ch = getchar();
        if(ch == '0'){
            ++a;
            sum[i] = sum[i - 1] - 1;
        }
        else if(ch == '1'){
            ++b;
            sum[i] = sum[i - 1] + 1;
        }
        if(l[sum[i] + N] || sum[i] == 0)
            ans1 = max(ans1, i - l[sum[i] + N]);
        else
            l[sum[i] + N] = i;
    }
    cout << ans1 << " " << 2 * min(a, b) << endl;
    return 0;
}