洛谷 2921 [USACO08DEC]在农场万圣节Trick or Treat on the Farm

【题解】

　　就是基环外向树森林找环，然后从环向外统计size就可以了。

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<vector>
 #define LL long long
 #define rg register
 #define N 200010
 using namespace std;
 int n,color,top,to[N],siz[N],st[N],pos[N];
 bool v[N],c[N];
 vector<int>from[N];
 inline int read(){
     int k=,f=; char c=getchar();
     while(c<''||c>'')c=='-'&&(f=-),c=getchar();
     while(''<=c&&c<='')k=k*+c-'',c=getchar();
     return k*f;
 }
 void calc(int x){
     v[x]=;
     for(rg int i=;i<from[x].size();i++)if(!siz[from[x][i]]){
         siz[from[x][i]]=siz[x]+; calc(from[x][i]);
     }
 }
 void dfs(int x){
     v[x]=; st[pos[x]=++top]=x;
     if(pos[to[x]]){
         int Size=top-pos[to[x]]+;
         for(rg int i=pos[to[x]];i<=top;i++) siz[st[i]]=Size,c[st[i]]=;
         for(rg int i=pos[to[x]];i<=top;i++) calc(st[i]);
 //        printf("%d %d\n",to[x],x);
         top=pos[to[x]]--;
     }
     else dfs(to[x]);
     top--; pos[x]=;
 }
 int main(){
     n=read();
     for(rg int i=;i<=n;i++) to[i]=read(),from[to[i]].push_back(i);
 //    for(rg int i=1;i<=n;i++){
 //        for(rg int j=0;j<from[i].size();j++) printf("%d ",from[i][j]); puts("");
 //    }
     for(rg int i=;i<=n;i++) if(!v[i]) dfs(i);
     for(rg int i=;i<=n;i++) printf("%d\n",siz[i]);
     return ;
 }