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[1005][1005];
int s1[1005][1005] = {0},e1[1005][1005] = {0},s2[1005][1005] = {0},e2[1005][1005] = {0};
int main()
{
    int n,m;
    cin >> n >> m;
    for(int i = 1;i <= n;i++)
    {
        for(int j = 1;j <= m;j++)   cin >> a[i][j];
    }
    s1[1][1] = a[1][1];
    s2[n][1] = a[n][1];
    e1[n][m] = a[n][m];
    e2[1][m] = a[1][m];
    for(int i = 2;i <= n;i++)
    {
        s1[i][1] += a[i][1]+s1[i-1][1];
        e2[i][m] += a[i][m]+e2[i-1][m];
    }
    for(int i = n-1;i >= 1;i--)
    {
        s2[i][1] += a[i][1]+s2[i+1][1];
        e1[i][m] += a[i][m]+e1[i+1][m];
    }
    for(int i = 2;i <= m;i++)
    {
        s1[1][i] += a[1][i]+s1[1][i-1];
        s2[n][i] += a[n][i]+s2[n][i-1];
    }
    for(int i = m-1;i >= 1;i--)
    {
        e2[1][i] += a[1][i]+e2[1][i+1];
        e1[n][i] += a[n][i]+e1[n][i+1];
    }
    for(int i = 2;i <= n;i++)
    {
        for(int j = 2;j <= m;j++)
        {
            s1[i][j] = max(s1[i-1][j]+a[i][j],s1[i][j-1]+a[i][j]);
        }
        for(int j = m-1;j >= 1;j--)
        {
            e2[i][j] = max(e2[i-1][j]+a[i][j],e2[i][j+1]+a[i][j]);
        }
    }
    for(int i = n-1;i >= 1;i--)
    {
        for(int j = 2;j <= m;j++)
        {
            s2[i][j] = max(s2[i][j-1]+a[i][j],s2[i+1][j]+a[i][j]);
        }
        for(int j = m-1;j >= 1;j--)
        {
            e1[i][j] = max(e1[i][j+1]+a[i][j],e1[i+1][j]+a[i][j]);
        }
    }
    int ans = 0;
    for(int i = 2;i < n;i++)
    {
        for(int j = 2;j < m;j++)
        {
            ans = max(ans,s1[i-1][j]+s2[i][j-1]+e1[i+1][j]+e2[i][j+1]);
            ans = max(ans,s1[i][j-1]+s2[i+1][j]+e1[i][j+1]+e2[i-1][j]);
        }
    }
    cout << ans << endl;
    return 0;

}