codeforces 839C

题目链接：https://codeforces.com/problemset/problem/839/C

dfs

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<cmath>
#include<stack>
#include<queue>
using namespace std;
typedef long long ll;

const int maxn = 100010;

int n;

int h[maxn],cnt = 0;
struct E{
	int to,next;
}e[maxn*2];
void add(int u,int v){
	e[++cnt].to = v;
	e[cnt].next = h[u];
	h[u] = cnt;
}

int deg[maxn],dep[maxn],sz[maxn];
double p[maxn],ans;

void dfs(int u,int par){
	sz[u] = 1;
	dep[u] = dep[par] + 1;
	if(par==1) p[u] = p[par] * (1.0 / deg[par]);
	else if(par!=0)p[u] = p[par] * (1.0 / (deg[par] - 1));
	for(int i=h[u];i!=-1;i=e[i].next){
		int v=e[i].to;
		if(v==par) continue;
		dfs(v,u);
		sz[u] += sz[v];
	}
	if(sz[u] == 1){
		ans += (dep[u]-1) * p[u];
	}
}

ll read(){ ll s=0,f=1; char ch=getchar(); while(ch<'0' || ch>'9'){ if(ch=='-') f=-1; ch=getchar(); } while(ch>='0' && ch<='9'){ s=s*10+ch-'0'; ch=getchar(); } return s*f; }

int main(){
	memset(h,-1,sizeof(h));
	n = read();
	ans = 0.0;
	int u,v;
	for(int i=1;i<n;++i){
		u = read(),v = read();
		add(u,v), add(v,u);
		++deg[u], ++deg[v];
	}
	
	p[1] = 1.0;
	dfs(1,0);
	
	printf("%.7f
",ans);
	
	return 0;
}