方格取数-CSP-J2020

#include <iostream>
#include <cstdio>
#define ll long long
using namespace std;
int n, m, a[1010][1010];
ll f[1010][1010][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]);

	for (int i = 1; i <= n; i++) {
		f[i][1][1] = f[i][1][0] = f[i][1][2] = f[i - 1][1][0] + a[i][1];
	}

	for (int j = 2; j <= m; j++) {
		//向下 向右
		f[1][j][0] = f[1][j - 1][2] + a[1][j];
		for (int i = 2; i <= n; i++)
			f[i][j][0] = max(f[i][j - 1][2], f[i - 1][j][0]) + a[i][j];
		//向上、向右
		f[n][j][1] = f[n][j - 1][2] + a[n][j];
		for (int i = n - 1; i >= 1; i--)
			f[i][j][1] = max(f[i][j - 1][2], f[i + 1][j][1]) + a[i][j];
		//合并
		for (int i = 1; i <= n; i++) {
			f[i][j][2] = max(f[i][j][0], f[i][j][1]);
		}
	}
	printf("%lld\n", f[n][m][2]);
	return 0;
}