RT，代码如下

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

int n , m ; 
int Map[1005][1005] ; 
long long f[1005] , up[1005] , set[1005];

int main()
{
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
  memset(up,0xc0,sizeof(up)) ;
  memset(set,0xc0,sizeof(set)) ;
  cin >> n >> m ;
  for ( int i = 1 ; i <= n ; i ++ ) {
    for ( int j = 1 ; j <= m ; j ++ ) {
      cin >> Map[i][j] ;
    }
  }
  for ( int i = 1 ; i <= n ; i ++) {
    f[i] = f[i-1] + Map[i][1] ;
  } 
  for ( int i = 2 ; i < m  ; i ++ ) {
    memset(up,0xc0,sizeof(up)) ;
    memset(set,0xc0,sizeof(set)) ;
    for ( int j = n ; j >= 1 ; j -- ) {
      up[j] =max(f[j],up[j+1])+Map[j][i];
    }
    for ( int j = 1 ; j <= n ; j ++ ) {
      set[j] = max (f[j], set[j-1]) + Map[j][i] ;
    }
    for ( int j = 1 ; j <= n ; j ++ ) {
      f[j] = max (up[j],set[j]) ;
    }
  }
  for ( int j = 1 ; j <= n ; j ++ ) {
    f[j] = max(f[j],f[j-1])+Map[j][m];
  }
  cout << f[n] ;
	return 0;
}