很简单的DP

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
const int NR=51;
int a[NR][NR],f[NR][NR][NR][NR];
int main()
{
	int n,m,i,j,k,l;
	cin>>m>>n;
	for(i=1;i<=m;i++)
		for(j=1;j<=n;j++) cin>>a[i][j];
	for(i=1;i<=m;i++)
		for(j=1;j<=n;j++)
			for(k=1;k<=m;k++)
				for(l=1;l<=n;l++)
				{
					f[i][j][k][l]=max(max(f[i-1][j][k-1][l],f[i-1][j][k][l-1]),max(f[i][j-1][k-1][l],f[i][j-1][k][l-1]));
					if(i==k && j==l) f[i][j][k][l]+=a[i][j];
					else f[i][j][k][l]+=a[i][j]+a[k][l];
				}
	cout<<f[m][n][m][n];
	return 0;
}