Ural_1146. Maximum Sum (DP)

　　/*题意是求最大子矩阵，表示线代没学好，好在不影响这道题。这题的主要思路就是二维化成一维。把二维数组前一列的元素累加到后边一列上，例如：

 0 -2 -7 0
 9 2 -6 2
-4 1 -4 1
-1 8 0 -2

处理后就是：
0  -2  -9  -9
9   11  5   7
-4 -3  -7  -6
-1  7   7   5

然后就是
dp[i][j] = max(dp[i][j-1], 0) + sum[j];

My Code:*/

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>

using namespace std;

const int N = 110;
const int inf = 0x7fffffff;

int sum[N][N];

int main() {
    //freopen("data.in", "r", stdin);

    int n, i, j, x, max, flag, k;
    while(~scanf("%d", &n)) {
        memset(sum, 0, sizeof(sum));
        for(i = 1; i <= n; i++) {
            for(j = 1; j <= n; j++) {
                scanf("%d", &x);
                sum[i][j] = sum[i][j-1] + x;
            }
        }
        max = -inf;
        for(i = 1; i <= n; i++) {
            for(j = 1; j <= i; j++) {
                for(flag = 0, k = 1; k <= n; k++) {
                    flag += sum[k][i] - sum[k][j-1];
                    if(flag > max)    max = flag;
                    if(flag < 0)    flag = 0;
                }
            }
        }
        printf("%d\n", max);
    }
    return 0;
}