AcWing 4405. 统计子矩阵(蓝桥杯13届（2022）B组)

思想：前缀和+双指针

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

const int N = 510;
typedef long long ll;

int n, m, k;
int s[N][N];
//前缀和
int main() {
    cin >> n >> m >> k;
    for (int i = 1; i <= n; i ++)
        for (int j = 1; j <= m; j ++) {
            cin >> s[i][j];
            s[i][j] += s[i - 1][j]; // 每列上的一维前缀和
        }

    ll res = 0;
//双指针
    for (int i = 1; i <= n; i ++) // i j 上下边界
        for (int j = i; j <= n; j ++)
            for (int l = 1, r = 1, sum = 0; r <= m; r ++) { // l r 左右边界
                sum += s[j][r] - s[i - 1][r];
                while (sum > k) {
                    sum -= s[j][l] - s[i - 1][l];
                    l ++;
                }
                res += r - l + 1;
            }

    cout << res;        
    return 0;
}