BZOJ 4726: [POI2017]Sabota? 树形dp

code: 

#include <cstdio> 
#include <string>  
#include <algorithm>     
#define N 500005 
#define inf 0.0000000001
using namespace std;   
namespace IO { 
    void setIO(string s) 
    {
        string in=s+".in"; 
        string out=s+".out"; 
        freopen(in.c_str(),"r",stdin); 
        // freopen(out.c_str(),"w",stdout) 
    }
};    
double f[N];   
double ans;        
int n,k,edges; 
int fa[N],hd[N],to[N],nex[N],size[N];  
void add(int u,int v) 
{
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v; 
}    
void dfs(int u) 
{    
    f[u]=-1.0;      
    size[u]=1; 
    for(int i=hd[u];i;i=nex[i]) 
    {
        int v=to[i]; 
        dfs(v);   
        size[u]+=size[v];  
    }    
    int mx=0;
    for(int i=hd[u];i;i=nex[i]) 
    {
        int v=to[i]; 
        f[u]=max(f[u],min(f[v],(double)size[v]/(size[u]-1)));            // 最大比例   
        mx=max(mx,size[v]); 
    }
    if(f[u]==-1.0)  f[u]=N;                              
    if(size[u]>k)   ans=max(ans,f[u]==N?1.0:f[u]);              
}
int main() 
{ 
    // IO::setIO("input");  
    int i,j;   
    scanf("%d%d",&n,&k); 
    for(i=2;i<=n;++i)  scanf("%d",&fa[i]),add(fa[i],i);   
    ans=0.0; 
    dfs(1);   
    if(ans==0.0000000) printf("0
");               
    else printf("%.10f
",ans); 
    return 0; 
}