90分求助

#include <bits/stdc++.h>
using namespace std;

const int MAXN = 1000, dx[] = {0, 1, -1}, dy[] = {-1, 0, 0};
int n, m, a[MAXN + 5][MAXN + 5];
long long f[MAXN + 5][MAXN + 5][3];

int main() {
    scanf("%d %d", &n, &m);
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= m; j++) {
            scanf("%d", &a[i][j]);
            f[i][j][0] = f[i][j][1] = f[i][j][2] = -1e17;
        }
    }
    f[1][1][0] = f[1][1][1] = f[1][1][2] = a[1][1];
    for (int i = 2; i <= n; i++) {
        f[i][1][1] = f[i - 1][1][1] + a[i][1];
    }
    for (int j = 2; j <= m; j++) {
        for (int i = 1; i <= n; i++) {
            f[i][j][0] = max(f[i][j - 1][0], max(f[i][j - 1][1], f[i][j - 1][2])) + a[i][j];
        }
        for (int i = 2; i <= n; i++) {
            f[i][j][1] = max(f[i - 1][j][0], f[i - 1][j][1]) + a[i][j];
        }
        for (int i = n - 1; i >= 1; i--) {
            f[i][j][2] = max(f[i + 1][j][0], f[i + 1][j][2]) + a[i][j];
        }
    }
    printf("%d", max(f[n][m][0], max(f[n][m][1], f[n][m][2])));
    return 0;
}