D. New Year Santa Network

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

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

 1 import java.util.ArrayList;
 2 import java.util.Scanner;
 3 
 4 public class Main {
 5 
 6     static int n;
 7     static long[] cnt = new long[(int) (1e5 + 50)];
 8     static ArrayList<Edge>[] road = new ArrayList[(int) (1e5 + 50)];
 9 
10     static int dfs(int x, int fa) {
11         int child = 1;
12         for (int i = 0, c, y; i < road[x].size(); i++) {
13             y = road[x].get(i).y;
14             if (y == fa) continue;
15             c = dfs(y, x);
16             child += c;
17             cnt[road[x].get(i).id] = 1L * c * (n - c);
18         }
19         return child;
20     }
21 
22     public static void main(String[] args) {
23         Scanner io = new Scanner(System.in);
24         n = io.nextInt();
25         for (int i = 0; i < road.length; i++) road[i] = new ArrayList<>();
26         int[] len = new int[n + 1];
27 
28         for (int i = 1, x, y; i <= n - 1; i++) {
29             x = io.nextInt();
30             y = io.nextInt();
31             len[i] = io.nextInt();
32             road[x].add(new Edge(y, i));
33             road[y].add(new Edge(x, i));
34         }
35 
36         dfs(1, -1);
37         double sum = 0;
38         for (int i = 1; i <= n - 1; i++) sum += cnt[i] * len[i];
39         int q = io.nextInt();
40         for (int i = 0, id2, len2; i < q; i++) {
41             id2 = io.nextInt();
42             len2 = io.nextInt();
43             sum -= cnt[id2] * (len[id2] - len2);
44             len[id2] = len2;
45             //什么时候检测到double就什么时候升double
46             //（）会改变检测顺序，所以下面的1.0如果没有就会溢出
47             System.out.println((sum * 6) / (1.0 * (n - 1) * n));
48         }
49     }
50 
51     static class Edge {
52         int y, id;
53 
54         public Edge(int y, int id) {
55             this.y = y;
56             this.id = id;
57         }
58     }
59 
60 }