hdu 1520 Anniversary party

http://acm.hdu.edu.cn/showproblem.php?pid=1520

#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
#define maxn 7000
using namespace std;

int dp[maxn][3];
int a[maxn],pre[maxn];
int n,u,v;
vector<int>g[maxn];

void dfs(int m)
{
    dp[m][1]=a[m];
    for(int i=0; i<(int)g[m].size(); i++)
    {
        int v2=g[m][i];
        dfs(v2);
        dp[m][0]+=(max(dp[v2][0],dp[v2][1]));
        dp[m][1]+=dp[v2][0];
    }
}

int main()
{
    while(scanf("%d",&n)!=EOF)
    {
        memset(a,0,sizeof(a));
        memset(pre,-1,sizeof(pre));
        memset(dp,0,sizeof(dp));
        for(int i=1; i<=n; i++)
        {
            g[i].clear();
        }
        for(int i=1; i<=n; i++)
        {
            scanf("%d",&a[i]);
        }
        int c;
        while(scanf("%d%d",&u,&v))
        {
            if(u==0&&v==0) break;
            c=u;
            g[v].push_back(u);
            pre[u]=v;
        }
        while(pre[c]!=-1)
        {
            c=pre[c];
        }
        dfs(c);
        int ans=max(dp[c][1],dp[c][0]);
        printf("%d
",ans);
    }
    return 0;
}