python/TangDou/CSP-J/J2_2020/4.cpp

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 1010;
const LL INF = 1e18;

LL f[N][N][3];
LL a[N][N];
int n, m;

int main() {
    cin >> n >> m;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++) {
            cin >> a[i][j];
            f[i][j][0] = f[i][j][1] = f[i][j][2] = -INF;
        }

    // 初始化
    f[1][1][0] = f[1][1][1] = 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][1], f[i][j - 1][0], 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];
    }

    cout << max(f[n][m][0], f[n][m][1]);

    // 1e3 * 1e3 * 1e4=1e10=10000000000>2147483647 因为 1e10是11位数，INT_MAX是10位数,肯定会超，要开long long
    return 0;
}