题解 | #最大子矩阵#前缀和模板题，记住就行。

//C++版代码
#include <iostream>
#include <vector>
#include <climits>
using namespace std;
int main() {
    int n;
    cin >> n;
    vector<vector<int>> prefix_sum(n + 1, vector<int>(n + 1));
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            int num;
            cin >> num;
            prefix_sum[i][j] = num + prefix_sum[i - 1][j] + prefix_sum[i][j - 1] - prefix_sum[i - 1][j - 1];
        }
    }
    int ans = INT_MIN;
    for (int x1 = 1; x1 <= n; x1++) {
        for (int y1 = 1; y1 <= n; y1++) {
            for (int x2 = x1; x2 <= n; x2++) {
                for (int y2 = y1; y2 <= n; y2++) {
                    int sum = prefix_sum[x2][y2] - prefix_sum[x1 - 1][y2] - prefix_sum[x2][y1 - 1] +
                              prefix_sum[x1 - 1][y1 - 1];
                    if (sum > ans) ans = sum;
                }
            }
        }
    }
    cout << ans << endl;
    return 0;
}
//Java版代码
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[][] prefix_sum = new int[n + 1][n + 1];
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++) {
                prefix_sum[i][j] = sc.nextInt() + prefix_sum[i - 1][j] + prefix_sum[i][j - 1] - prefix_sum[i - 1][j - 1];
            }
        }
        int ans = Integer.MIN_VALUE;
        for (int x1 = 1; x1 <= n; x1++) {
            for (int y1 = 1; y1 <= n; y1++) {
                for (int x2 = x1; x2 <= n; x2++) {
                    for (int y2 = y1; y2 <= n; y2++) {
                        int sum = prefix_sum[x2][y2] - prefix_sum[x2][y1 - 1] - prefix_sum[x1 - 1][y2] + prefix_sum[x1 - 1][y1 - 1];
                        if (sum > ans) ans = sum;
                    }
                }
            }
        }
        System.out.println(ans);
    }
}