HDU #2376. Average distance

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题意

问树上路径长度的期望值

题解

dfs求出每条边左右连接的点数，统计答案，最后*2/n/(n+1)

调试记录

无

#include <cstdio>
#include <cstring>
#define int long long
const int maxn = 1e4 + 5;
using namespace std;

struct node{
	int to, next, l;
}e[maxn << 1];
int head[maxn], tot;
void addedge(int u, int v, int l){
	e[++tot].to = v, e[tot].next = head[u], e[tot].l = l;
	head[u] = tot;
}

long long ans; int size[maxn], n;
void dfs(int cur, int fa){
	size[cur] = 1;
	for (int i = head[cur]; i; i = e[i].next){
		if (e[i].to != fa){
			dfs(e[i].to, cur);
			size[cur] += size[e[i].to];
			ans += e[i].l * size[e[i].to] * (n - size[e[i].to]);
		}
	}
}

int T;
signed main(){
	scanf("%lld", &T);
	while (T--){
		scanf("%lld", &n);
		memset(e, 0, sizeof e);
		memset(head, 0, sizeof head);
		memset(size, 0, sizeof size);
		tot = 0; ans = 0;
		for (int u, v, l, i = 1; i < n; i++){
			scanf("%lld%lld%lld", &u, &v, &l); u++, v++;
			addedge(u, v, l), addedge(v, u, l);
		}
		dfs(1, 0);
		printf("%.6lf\n", 2.0 * ans / n / (n - 1));
	}
	
	return 0;
}