考场暴力写法 $O(n^2m)$ 预计会炸

for(int i = 1 ; i <= n ; i ++)
	for(int j = 1 ; j <= m ; j ++)
    	s[i][j] = map[i][j]+s[i-1][j];
for(int j = 1 ; j <= m ; j ++)
	for(int i = 1 ; i <= n ; i ++){
    	for(int k = 1 ; k < i ; k ++)
        f[i][j] = max(f[i][j],f[k][i-1]+s[i][j]-s[k-1][j];
      f[i][j] = max(f[i][j],f[i][j-1]+map[i][j];
      for(int k = i+1 ; k <= n ; k ++)
        f[i][j] = max(f[i][j],f[k][i-1]+s[k][j]-s[i-1][j];
  	}

听谷群大佬说这题可以做到n^2，怎么写啊