[[http://poj.org/problem?id=2057|POJ 2057]]

//不太聪明的版本//

<code cpp>
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<cstdlib>
#define INF 0x3f3f3f3f
using namespace std;
int n,f[1005],sz[1005],head[1005],mxlen[1005],cnt;
bool worm[1005];
struct Node{int nxt,to;}Edges[1005*2];
void addedge(int u,int v)
{
	Edges[++cnt].nxt=head[u];
	head[u]=cnt,Edges[cnt].to=v;
}
int dfs(int u)
{
	if(head[u]==-1){sz[u]=1;return f[u];}
	if(f[u])return f[u];
	int son[8],top=0,p[8]={0,1,2,3,4,5,6,7};
	for(int i=head[u];~i;i=Edges[i].nxt)
	{
		int v=Edges[i].to;
		dfs(v);
		sz[u]+=sz[v],son[top++]=v;
	}
	f[u]=INF;
	do
	{
		int len=0,res=0;
		for(int i=0;i<top;i++)
		{
			int v=son[p[i]];
			len+=i?2:1;
			res+=dfs(v)+sz[v]*len;
			if(!worm[v])len+=mxlen[v];
		}
		len++;
		if(res<f[u]||(res==f[u]&&len<mxlen[u]))
		f[u]=res,mxlen[u]=len;
	}
	while(next_permutation(p,p+top));
	return f[u];
}
int main()
{
	while(~scanf("%d",&n))
	{
		if(!n)break;
		memset(f,0,sizeof(f));
		memset(sz,0,sizeof(sz));
		memset(head,-1,sizeof(head));
		memset(worm,0,sizeof(worm));
		memset(mxlen,0,sizeof(mxlen));
		cnt=0;
		for(int i=1;i<=n;i++)
		{
			int f;
			char c;
			scanf("%d %c",&f,&c);
			if(c=='Y')worm[i]=1;
			if(f>0)addedge(f,i);
		}
		printf("%.4lf\n",1.0*dfs(1)/sz[1]);
	}
	return 0;
}
</code>

//贪心//

<code cpp>
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<cstdlib>
#define INF 0x3f3f3f3f
using namespace std;
int n,f[1005],sz[1005],head[1005],mxlen[1005],cnt;
bool worm[1005];
struct Node{int nxt,to;}Edges[1005*2];
void addedge(int u,int v)
{
	Edges[++cnt].nxt=head[u];
	head[u]=cnt,Edges[cnt].to=v;
}
bool cmp(int x,int y){return (mxlen[x]+2)*sz[y]<(mxlen[y]+2)*sz[x];}
int dfs(int u)
{
	if(head[u]==-1){sz[u]=1;return f[u];}
	if(f[u])return f[u];
	int son[8],top=0;
	for(int i=head[u];~i;i=Edges[i].nxt)
	{
		int v=Edges[i].to;
		dfs(v);
		sz[u]+=sz[v],son[top++]=v;
		mxlen[u]+=mxlen[v]+2;
	}
	if(worm[u])mxlen[u]=0;
	f[u]=0;
	sort(son,son+top,cmp);
	for(int i=0;i<top;i++)
	{
		f[u]+=f[son[i]]+sz[son[i]];
		for(int j=0;j<i;j++)
		f[u]+=(mxlen[son[j]]+2)*sz[son[i]];
	}
	return f[u];
}
int main()
{
	while(~scanf("%d",&n))
	{
		if(!n)break;
		memset(f,0,sizeof(f));
		memset(sz,0,sizeof(sz));
		memset(head,-1,sizeof(head));
		memset(worm,0,sizeof(worm));
		memset(mxlen,0,sizeof(mxlen));
		cnt=0;
		for(int i=1;i<=n;i++)
		{
			int f;
			char c;
			scanf("%d %c",&f,&c);
			if(c=='Y')worm[i]=1;
			if(f>0)addedge(f,i);
		}
		printf("%.4lf\n",1.0*dfs(1)/sz[1]);
	}
	return 0;
}
</code>