HDU 2376 Average distance（树上两点间的期望）

#include<stdio.h>
#include<string.h>
#include<algorithm>
using namespace std;
const int maxn=20005;
struct node
{
    int x,y;
    int v,next;
}edge[maxn];
int head[maxn],cnt,n;
int sz[maxn];
double an;
int add(int x,int y,int v)
{
    edge[cnt].x=x;
    edge[cnt].y=y;
    edge[cnt].v=v;
    edge[cnt].next=head[x];
    head[x]=cnt++;
}
int dfs(int root,int fa)
{
    sz[root]=1;
    for(int i=head[root];i!=-1;i=edge[i].next)
    {
        int a=edge[i].y;
        int b=edge[i].v;
        if(a==fa)   continue;
        dfs(a,root);
        sz[root]=sz[root]+sz[a];
        an=an+(double)(n-sz[a])*sz[a]*b;
    }
}
int main()
{
    int t;  scanf("%d",&t);
    while(t--)
    {
        memset(head,-1,sizeof(head));
        memset(sz,0,sizeof(sz));
        cnt=0;
        scanf("%d",&n);
        for(int i=0;i<n-1;i++)
        {
            int x,y,z;  scanf("%d%d%d",&x,&y,&z);
            add(x,y,z); add(y,x,z);
        }
        an=0.0;
        dfs(0,-1);
        printf("%.7f\n",an/ (1.0 * n * (n - 1) / 2.0));
    }
}

Average distance

Time Limit: 1000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 943    Accepted Submission(s): 318
Special Judge

Problem Description

Given a tree, calculate the average distance between two vertices in the tree. For example, the average distance between two vertices in the following tree is (d ₀₁ + d ₀₂ + d ₀₃ + d ₀₄ + d ₁₂ +d ₁₃ +d ₁₄ +d ₂₃ +d ₂₄ +d ₃₄)/10 = (6+3+7+9+9+13+15+10+12+2)/10 = 8.6.

 

Input

On the first line an integer t (1 <= t <= 100): the number of test cases. Then for each test case: 

One line with an integer n (2 <= n <= 10 000): the number of nodes in the tree. The nodes are numbered from 0 to n - 1. 

n - 1 lines, each with three integers a (0 <= a < n), b (0 <= b < n) and d (1 <= d <= 1 000). There is an edge between the nodes with numbers a and b of length d. The resulting graph will be a tree. 

 

Output

For each testcase: 

One line with the average distance between two vertices. This value should have either an absolute or a relative error of at most 10 ^-6

 

Sample Input

  
  

   
   1
5
0 1 6
0 2 3
0 3 7
3 4 2
  
  

 

Sample Output

  
  

   
   8.6
  
  

 

Source

bapc2007

 

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