poj 2342

经典dp。。不多说啦。因为他是n-1条边，所以不用考虑森林的情况

# include <iostream>
# include <cstdio>
# include <algorithm>
# include <cstring>
# include <vector>
# define inf -0x7ffffff
using namespace std;
typedef struct NODE{
    int k;struct NODE* next;
}Node;
Node edge[7000],*tail=edge,*G[7000];
void add(int a,int b)
{
    tail->k = b;tail->next = G[a];G[a] = tail++;
}
int v[7000],vis[7000],d[7000][2];
int dp(int u,int f)
{
    if (d[u][f]!=inf) return d[u][f];
    Node* i;
    if (f)
        d[u][f] = v[u];
    else
        d[u][f] = 0;
    if (f)
        for (i=G[u];i;i=i->next)
            d[u][f] += dp(i->k,0);
    else
        for (i=G[u];i;i=i->next)
            d[u][f] += max(dp(i->k,0),dp(i->k,1));
    return d[u][f];
}
int main()
{
    int n,i,a,b,t;
    scanf("%d",&n);
    for (i=0;i<7000;++i)
    {
        d[i][0] = inf;
        d[i][1] = inf;
    }
    for (i=1;i<=n;++i)
        scanf("%d",&v[i]);
    for (i=0;i<n-1;++i)
    {
        scanf("%d%d",&a,&b);
        add(b,a);
        vis[a] = 1;
    }
    scanf("%d%d",&a,&b);
    for (i=1;i<=n;++i)
        if (!vis[i])
            t = i;
    printf("%d
",max(dp(t,1),dp(t,0)));
    return 0;
}