275. 传纸条
     DP 三维数组。

#include <iostream>
#include <cstring>
using namespace std;
const int maxn = 100;
int f[maxn][maxn][maxn];//第一维是步数，第二维是x1，第三维是x2。
int w[maxn][maxn];//y可以用x表示
int main()
{int n, m;cin >> n >> m;for (int i = 1; i <= n; i++)for (int j = 1; j <= m; j++)cin >> w[i][j];for (int k = 2; k <= n + m; k++){for (int x1 = max(1, k - m); x1 <= min(n, k - 1); x1++){for (int x2 = max(1, k - m); x2 <= min(n, k - 1); x2++){int t = w[x1][k - x1];if (x1 != x2)t += w[x2][k - x2];for (int i = 0; i < 2; i++){for (int j = 0; j < 2; j++){f[k][x1][x2] = max(f[k][x1][x2], f[k - 1][x1 - i][x2 - j] + t);}}}}}cout << f[n + m][n][n] << endl;
}