C++ 代码

#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

typedef long long LL;

const int N = 1010;
int n, m;
int w[N][N];
LL f[N][N];

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    cin >> n >> m;
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= m; j++)
            cin >> w[i][j];

    memset(f, -0x3f, sizeof f);
    f[1][0] = 0;

    for(int j = 1; j <= m; j++){
        LL s = -1e18;
        for(int i = 1; i <= n; i++){
            s = max(s, f[i][j - 1]) + w[i][j];
            f[i][j] = max(s, f[i][j]);
        }
        s = -1e18;
        for(int i = n; i >= 0; i--){
            s = max(s, f[i][j - 1]) + w[i][j];
            f[i][j] = max(s, f[i][j]);
        }
    }

    cout << f[n][m] << endl;

    return 0;
}