D. New Year Santa Network

http://codeforces.com/contest/500/problem/D

https://blog.csdn.net/ShiAokai/article/details/42921885?locationNum=8&fps=1

import java.util.ArrayList;
import java.util.Scanner;

public class Main {

    static int n;
    static long[] cnt = new long[(int) (1e5 + 50)];
    static ArrayList<Edge>[] road = new ArrayList[(int) (1e5 + 50)];

    static int dfs(int x, int fa) {
        int child = 1;
        for (int i = 0, c, y; i < road[x].size(); i++) {
            y = road[x].get(i).y;
            if (y == fa) continue;
            c = dfs(y, x);
            child += c;
            cnt[road[x].get(i).id] = 1L * c * (n - c);
        }
        return child;
    }

    public static void main(String[] args) {
        Scanner io = new Scanner(System.in);
        n = io.nextInt();
        for (int i = 0; i < road.length; i++) road[i] = new ArrayList<>();
        int[] len = new int[n + 1];

        for (int i = 1, x, y; i <= n - 1; i++) {
            x = io.nextInt();
            y = io.nextInt();
            len[i] = io.nextInt();
            road[x].add(new Edge(y, i));
            road[y].add(new Edge(x, i));
        }

        dfs(1, -1);
        double sum = 0;
        for (int i = 1; i <= n - 1; i++) sum += cnt[i] * len[i];
        int q = io.nextInt();
        for (int i = 0, id2, len2; i < q; i++) {
            id2 = io.nextInt();
            len2 = io.nextInt();
            sum -= cnt[id2] * (len[id2] - len2);
            len[id2] = len2;
            //什么时候检测到double就什么时候升double
            //（）会改变检测顺序，所以下面的1.0如果没有就会溢出
            System.out.println((sum * 6) / (1.0 * (n - 1) * n));
        }
    }

    static class Edge {
        int y, id;

        public Edge(int y, int id) {
            this.y = y;
            this.id = id;
        }
    }

}