BZOJ 4726: [POI2017]Sabota?

/**************************************************************
    Problem: 4726
    User: BeiYu
    Language: C++
    Result: Accepted
    Time:12456 ms
    Memory:37616 kb
****************************************************************/
 
#include <cstdio>
#include <vector>
#include <iostream>
using namespace std;
 
#define debug(a) cout<<#a<<"="<<a<<endl
const int N = 500005;
 
int n,k;
vector< int > g[N];
int sz[N];
double f[N];
double ans;
 
inline int in(int x=0,char ch=getchar()){ while(ch>‘9‘ || ch<‘0‘) ch=getchar();
    while(ch>=‘0‘ && ch<=‘9‘) x=x*10+ch-‘0‘,ch=getchar();return x; }
 
void DFS(int u,int fa){
    sz[u]=1;for(int i=0,lim=g[u].size(),v;i<lim;i++) v=g[u][i],DFS(v,u),sz[u]+=sz[v];
}
void DP(int u,int fa){
    if(sz[u]==1){ f[u]=1.0;return; }
    for(int i=0,lim=g[u].size(),v;i<lim;i++) {
        v=g[u][i],DP(v,u),f[u]=max(f[u],min(f[v],(double)sz[v]/(sz[u]-1)));
    }if(sz[u]>k) ans=max(ans,f[u]);
}
int main(){
    n=in(),k=in();
    for(int i=2,u;i<=n;i++) u=in(),g[u].push_back(i);
    DFS(1,0);
    DP(1,0);
    printf("%.8lf\n",ans);
    return 0;
}