Codeforces_429_B

http://codeforces.com/problemset/problem/429/B

挺简单的题，先求出四个点到每一点的最大和，然后枚举每一点，取和最大值。

注意两条路相交的点有且只有一个，这点的数值不加。

#include<iostream>
#include<algorithm>
using namespace  std;
int a[][];
int s1[][] = {},e1[][] = {},s2[][] = {},e2[][] = {};
int main()
{
    int n,m;
    cin >> n >> m;
    for(int i = ;i <= n;i++)
    {
        for(int j = ;j <= m;j++)   cin >> a[i][j];
    }
    s1[][] = a[][];
    s2[n][] = a[n][];
    e1[n][m] = a[n][m];
    e2[][m] = a[][m];
    for(int i = ;i <= n;i++)
    {
        s1[i][] += a[i][]+s1[i-][];
        e2[i][m] += a[i][m]+e2[i-][m];
    }
    for(int i = n-;i >= ;i--)
    {
        s2[i][] += a[i][]+s2[i+][];
        e1[i][m] += a[i][m]+e1[i+][m];
    }
    for(int i = ;i <= m;i++)
    {
        s1[][i] += a[][i]+s1[][i-];
        s2[n][i] += a[n][i]+s2[n][i-];
    }
    for(int i = m-;i >= ;i--)
    {
        e2[][i] += a[][i]+e2[][i+];
        e1[n][i] += a[n][i]+e1[n][i+];
    }
    for(int i = ;i <= n;i++)
    {
        for(int j = ;j <= m;j++)
        {
            s1[i][j] = max(s1[i-][j]+a[i][j],s1[i][j-]+a[i][j]);
        }
        for(int j = m-;j >= ;j--)
        {
            e2[i][j] = max(e2[i-][j]+a[i][j],e2[i][j+]+a[i][j]);
        }
    }
    for(int i = n-;i >= ;i--)
    {
        for(int j = ;j <= m;j++)
        {
            s2[i][j] = max(s2[i][j-]+a[i][j],s2[i+][j]+a[i][j]);
        }
        for(int j = m-;j >= ;j--)
        {
            e1[i][j] = max(e1[i][j+]+a[i][j],e1[i+][j]+a[i][j]);
        }
    }
    int ans = ;
    for(int i = ;i < n;i++)
    {
        for(int j = ;j < m;j++)
        {
            ans = max(ans,s1[i-][j]+s2[i][j-]+e1[i+][j]+e2[i][j+]);
            ans = max(ans,s1[i][j-]+s2[i+][j]+e1[i][j+]+e2[i-][j]);
        }
    }
    cout << ans << endl;
    return ;

}