暴力：复杂度 $O(mn^2)$

二维差分+前缀和：复杂度 $O(m+n^2)$

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 1e3 + 10, INF = 0x3f3f3f3f;
int n, m, d[N][N];

int brute_force() {
    cin >> n >> m;
    int x1, y1, x2, y2;
    while (m--) {
        cin >> x1 >> y1 >> x2 >> y2;
        for (int i = x1; i <= x2; i++)
            for (int j = y1; j <= y2; j++)
                d[i][j]++;
    }
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            cout << d[i][j] << " \n"[j == n];
    return 0;
}
int solve_ac() {
    cin >> n >> m;
    int x1, y1, x2, y2;
    while (m--) {
        cin >> x1 >> y1 >> x2 >> y2;
        d[x1][y1]++;
        d[x1][y2 + 1]--;
        d[x2 + 1][y1]--;
        d[x2 + 1][y2 + 1]++;
    }
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++) {
            d[i][j] += d[i - 1][j] + d[i][j - 1] - d[i - 1][j - 1];
            cout << d[i][j] << " \n"[j == n];
        }
    return 0;
}
int main() {
    brute_force();
    solve_ac();
}