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=;
 struct Edge{LL to,next;}edge[Maxn<<];
 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!=-;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]=;
     for (LL i=head[u];i!=-;i=edge[i].next)
     {
         if (i==Ban || (i^)==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,-,sizeof(head)); cnt=;
     for (LL i=;i<=n;i++)
     {
         LL u;
         scanf("%lld%lld",&a[i],&u);
         Add(u,i),Add(i,u);
     }

     for (LL i=;i<=n;i++)
         if (!vis[i])
         {
             Dfs(i,);
             Dp(U,,E);
             LL Ret=G[U];
             Dp(V,,E);
             Ret=Max(Ret,G[V]);
             Ans+=Ret;
         }
     printf("%lld\n",Ans);
     return ;
 }

C++