题解：贪心，每次选深度最大的未被覆盖节点，在他的2级祖先处放一个
不会DP做法QWQ
#include<iostream> #include<cstdio> #include<cstring> #include<algorithm> using namespace std; const int maxn=100009; int n; int ans=0; int cntedge; int head[maxn]; int to[maxn<<1],nex[maxn<<1]; void Addedge(int x,int y){ nex[++cntedge]=head[x]; to[cntedge]=y; head[x]=cntedge; } int father[maxn]; int dep[maxn]; int vis[maxn]; void Minit(){ memset(head,0,sizeof(head)); memset(dep,0,sizeof(dep)); memset(father,0,sizeof(father)); memset(vis,0,sizeof(vis)); cntedge=0; } void Dfs(int now,int fa){ father[now]=fa; dep[now]=dep[fa]+1; for(int i=head[now];i;i=nex[i]){ if(to[i]==fa)continue; Dfs(to[i],now); } } void C(int x,int fa,int d){ vis[x]=1; if(d==0)return; for(int i=head[x];i;i=nex[i]){ if(to[i]==fa)continue; C(to[i],x,d-1); } } int p[maxn]; int cmp(const int &rhs1,const int &rhs2){ return dep[rhs1]>dep[rhs2]; } int main(){ scanf("%d",&n); Minit(); for(int i=2;i<=n;++i){ int x;scanf("%d",&x); Addedge(x,i); Addedge(i,x); } Dfs(1,0); for(int i=1;i<=n;++i)p[i]=i; sort(p+1,p+1+n,cmp); for(int i=1;i<=n;++i){ if(vis[p[i]])continue; ++ans; if(father[father[p[i]]]==0)break; C(father[father[p[i]]],0,2); } cout<<ans<<endl; return 9; }