CF——Codeforces Round #428 (Div. 2)C. Journey

题目：

[C. Journey](http://codeforces.com/contest/839/problem/C)

题解：

#include <bits/stdc++.h>
#define maxn 100000 + 10
typedef long long LL;
using namespace std;
int t = 0;
int n;
double ans = 0;
vector <int> a[maxn];

void dfs(int y,int x,int s,double p)
{

if(a[x].size() <= 1&&x != 1)
{ans += double(s*1.0)*p;return;}
int len =a[x].size();
for(int i = 0; i < len; i++)
if(a[x][i] != y)
if(x!=1) dfs(x,a[x][i],s+1,p*(double)(1.0)/(a[x].size()-1));
else dfs(x,a[x][i],s+1,p*(double)(1.0)/a[x].size());

}

int main()
{
cin >> n;
int u,v;
for(int i = 1; i < n; i++)
{
scanf("%d%d",&u,&v);
a[u].push_back(v);
a[v].push_back(u);
}
dfs(0,1,0,1);

printf("%.15lf\n",ans);
return 0;
}