day 2 下午 骑士 基环树+树形DP

 #include<iostream>
 #include<cstdio>
 #include<cstring>
 #include<cstdlib>
 #include<algorithm>
 using namespace std;
 const int maxn=2e6+;
 long long  dp[maxn][];
 long long  w[maxn];

 struct node
 {
     long long  to,next,value;
 }way[maxn];
 long long tmp;
 long long tot;
 long long  n,x;
 long long  head[maxn];
 bool vis[maxn];
 long long  root;
 long long  father;
 long long  ans1,ans;
 int  read()
 {
     int flag = ;
     char c = getchar();
     while ((!isdigit(c)) && c != '-') c = getchar();
     if (c == '-') flag = -, c = getchar();
     int init = c & ;
     while (isdigit(c = getchar())) init = (init << ) + (init << ) + (c & );
     return init * flag;
 }
 void add(long long  x,long long  y,long long  w)
 {
     way[++tot].next=head[x];
     way[tot].to=y;
     way[tot].value=w;
     head[x]=tot;
 }
 void  dfs(long long  x,long long  fa)
 {
     vis[x]=;
     for(int i=head[x];i;i=way[i].next)
     {
         long long  to=way[i].to;
         if(to==fa)
         {
             continue;
         }
         if(vis[to])
         {
             root=to;
             father=x;
             tmp=way[i].value;
         }
         else
         {
             dfs(to,x);
         }
     }
 }
 void  shu_dp(long long  x,long long  fa)
 {
     dp[x][]=;
     dp[x][]=w[x];
     for(int i=head[x];i;i=way[i].next)
     {
         long long  to=way[i].to;
         if(way[i].value==tmp||to==fa)
         {
             continue;
         }
         shu_dp(to,x);
         dp[x][]+=max(dp[to][],dp[to][]);
         dp[x][]+=dp[to][];
     }
 }
 int main()
 {
     n=read();
     for(int i=;i<=n;i++)
     {
         w[i]=read();
         x=read();
         add(i,x,i);
         add(x,i,i);
     }
     for(int i=;i<=n;i++)
     {
         if(!vis[i])
         {
             dfs(i,);
             shu_dp(root,);
             ans1=dp[root][];
             shu_dp(father,);
             long long ans2;
             ans2=dp[father][];
             ans1=max(ans1,ans2);
             ans+=ans1;
         }
     }
     printf("%lld\n",ans);
     return ;
 }