hdu 4607 Park Visit （树的直径）

题目：

Problem Description

Claire and her little friend, ykwd, are travelling in Shevchenko's Park! The park is beautiful - but large, indeed. N feature spots in the park are connected by exactly (N-1) undirected paths, and Claire is too tired to visit all of them. After consideration, she decides to visit only K spots among them. She takes out a map of the park, and luckily, finds that there're entrances at each feature spot! Claire wants to choose an entrance, and find a way of visit to minimize the distance she has to walk. For convenience, we can assume the length of all paths are 1.
Claire is too tired. Can you help her?

Input

An integer T(T≤20) will exist in the first line of input, indicating the number of test cases.
Each test case begins with two integers N and M(1≤N,M≤10⁵), which respectively denotes the number of nodes and queries.
The following (N-1) lines, each with a pair of integers (u,v), describe the tree edges.
The following M lines, each with an integer K(1≤K≤N), describe the queries.
The nodes are labeled from 1 to N.

Output

For each query, output the minimum walking distance, one per line.

Sample Input

4 2

3 2

1 2

4 2

Sample Output

思路：

求一条最长的路（树的直径）。如果问的点比最长的路上的点还多，则多一个点就多走两条路。

代码：

两次搜索：

 1 #include<stdio.h>
 2 #include<string.h>
 3 #include<vector>
 4 #include<iostream>
 5 using namespace std;
 6 
 7 const int T=100005;
 8 vector < int > Q[T];
 9 int flag[T],V,Max;
10 
11 
12 void dfs(int x,int xx)
13 {
14     flag[x]=1;
15     for(int i=0;i<(int)Q[x].size();i++)
16     {
17         int k=Q[x][i];
18         if(!flag[k])
19         {
20             dfs(k,xx+1);
21             flag[k]=0;
22         }
23     }
24     if(xx>Max)
25     {
26         V=x;
27         Max=xx;
28     }
29 }
30 
31 
32 int main()
33 {
34     int N;
35     scanf("%d",&N);
36     while(N--)
37     {
38         int n,m,i,a,b,v;
39         memset(flag,0,sizeof(flag));
40         scanf("%d %d",&n,&m);
41         for(i=1;i<=n;i++)
42            Q[i].clear();
43         for(i=1;i<n;i++)
44         {
45             scanf("%d %d",&a,&b);
46             Q[a].push_back(b);
47             Q[b].push_back(a);
48         }
49         V=0; Max=0;
50         dfs(1,1);
51         flag[1]=0;
52         v=V; Max=0; V=0;
53         dfs(v,1);
54        // printf("Max==%d
",Max);
55         for(i=1;i<=m;i++)
56         {
57             scanf("%d",&a);
58             if(a<=Max) printf("%d
",a-1);
59             else printf("%d
",Max-1+(a-Max)*2);
60         }
61     }
62 }

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树形dp：

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