激光炸弹

https://www.acwing.com/problem/content/101/

#include <bits/stdc++.h>

using namespace std;
const int maxn = 5e3 + 10;
int N, R, x, y, z, a[maxn][maxn], r, c, ans;

int main() {
    cin >> N >> R;
    r = c = R;
    for (int i = 1; i <= N; i++) {
        cin >> x >> y >> z;
        x++;
        y++;
        a[x][y] = z;
        r = max(r, x);
        c = max(c, y);
    }
    for (int i = 1; i <= r; i++) {
        for (int j = 1; j <= c; j++) {
            a[i][j] += a[i - 1][j] + a[i][j - 1] - a[i - 1][j - 1];
        }
    }
    for (int i = R; i <= r; i++) {
        for (int j = R; j <= c; j++) {
            ans = max(ans, a[i][j] - a[i - R][j] - a[i][j - R] + a[i - R][j - R]);
        }
    }
    cout << ans << endl;
    return 0;
}

为什么要++