求解多个 $r×r$ 的矩阵和，利用二维前缀和。

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 5e3 + 5;
int t, n = 5001, r, g[N][N], ans;
int main() {
    cin >> t >> r;
    int x, y, z;
    for (int i = 1; i <= t; i++)
        cin >> x >> y >> z, g[x + 1][y + 1] += z;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            g[i][j] += g[i - 1][j] + g[i][j - 1] - g[i - 1][j - 1];
    for (int i = r; i <= n; i++)
        for (int j = r; j <= n; j++) {
            int s = g[i][j] - g[i - r][j] - g[i][j - r] + g[i - r][j - r];
            ans = max(ans, s);
        }
    cout << ans << endl;
}