CF839C树形DP期望

给一颗树，求从根出发路径长度的期望是多少。

树形DP 要想清楚期望的计算

/** @Date    : 2017-08-12 23:09:41
  * @FileName: C.cpp
  * @Platform: Windows
  * @Author  : Lweleth (SoungEarlf@gmail.com)
  * @Link    : https://github.com/
  * @Version : $Id$
  */
#include 
#define LL long long
#define PII pair
#define MP(x, y) make_pair((x),(y))
#define fi first
#define se second
#define PB(x) push_back((x))
#define MMG(x) memset((x), -1,sizeof(x))
#define MMF(x) memset((x),0,sizeof(x))
#define MMI(x) memset((x), INF, sizeof(x))
using namespace std;

const int INF = 0x3f3f3f3f;
const int N = 1e5+20;
const double eps = 1e-6;


vectoredg[N];
int vis[N];
double f[N];
double dfs(int x, int pre)
{
int flag = 0;
for(auto i: edg[x])
{
if(i == pre)
continue;
flag++;
dfs(i, x);
f[x] += f[i];
}
if(flag)
f[x] = 1.0*f[x]/flag + 1;
}

int main()
{
int n;
while(cin >> n)
{
MMF(vis);
for(int i = 0; i <= n; i++)
edg[i].clear();
for(int i = 0; i {
int x, y;
scanf("%d%d", &x, &y);
edg[x].PB(y);
edg[y].PB(x);
}
dfs(1, -1);
//cout <//printf("%.10lf\n", (1.0000000*ans/cnt));
printf("%.10lf\n", f[1]);
}

    return 0;
}