BZOJ 1040 树形DP+环套树

就是有n个点n条边，那么有且只有一个环那么用Dfs把在环上的两个点找到。然后拆开，从这条个点分别作树形Dp即可.

 
#include <cstdio>
#include <cstring>
#define LL long long
const LL Maxn=1001000;
struct Edge{LL to,next;}edge[Maxn<<2];
LL head[Maxn],F[Maxn],G[Maxn],cnt,U,V,E,vis[Maxn],a[Maxn],n,Ans;
inline void Add(LL u,LL v) {edge[++cnt].to=v;edge[cnt].next=head[u];head[u]=cnt;}
inline LL Max(LL x,LL y) {return x>y?x:y;}
void Dfs(LL u,LL fa)
{
    vis[u]=true;
    for (LL i=head[u];i!=-1;i=edge[i].next)
    {
        if (edge[i].to==fa) continue;
        if (vis[edge[i].to])
        {
            U=u,V=edge[i].to;
            E=i;
            continue;
        }
        Dfs(edge[i].to,u);
    }
}
void Dp(LL u,LL fa,LL Ban)
{
    F[u]=a[u],G[u]=0;
    for (LL i=head[u];i!=-1;i=edge[i].next)
    {
        if (i==Ban || (i^1)==Ban || edge[i].to==fa) continue;
        Dp(edge[i].to,u,Ban);
        F[u]+=G[edge[i].to];
        G[u]+=Max(F[edge[i].to],G[edge[i].to]);
    }
}
 
int main()
{
    scanf("%lld",&n);
    memset(head,-1,sizeof(head)); cnt=1;
    for (LL i=1;i<=n;i++) 
    {
        LL u;
        scanf("%lld%lld",&a[i],&u);
        Add(u,i),Add(i,u);
    }
     
    for (LL i=1;i<=n;i++)
        if (!vis[i])
        {
            Dfs(i,0);
            Dp(U,0,E);
            LL Ret=G[U];
            Dp(V,0,E);
            Ret=Max(Ret,G[V]);
            Ans+=Ret;
        }
    printf("%lld
",Ans);
    return 0;
}