hdu 4607 Park Visit 求树的直径

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=4607

题目大意：给你n个点，n-1条边，将图连成一棵生成树，问你从任意点为起点，走k（k<=n)个点，至少需要走多少距离（每条边的距离是1）；

思路：树形dp求树的直径r；

a：若k<=r+1 ，ans = k-1；

b：若k>=r+1，ans = r+(k-（r+1))*2；

代码：

 #include "stdio.h"
 #include "string.h"

 #define N 110000

 struct node
 {
     int x,y;
     bool visit;
     int next;
 } edge[*N];
 int idx,head[N];

 inline int MAX(int a,int b)
 {
     return a>b?a:b;
 }
 void Init()
 {
     idx=;
     memset(head,-,sizeof(head));
 }
 void Add(int x,int y)
 {
     edge[idx].x=x;
     edge[idx].y=y;
     edge[idx].visit=false;
     edge[idx].next=head[x];
     head[x]=idx++;
 }

 int n;
 int maxn[N],smaxn[N];

 void DFS(int x)
 {
     int i,y;
     maxn[x] = smaxn[x] = ;
     for(i=head[x]; i!=-; i=edge[i].next)
     {
         y = edge[i].y;
         if(edge[i].visit) continue;
         edge[i].visit = edge[i^].visit = true;
         DFS(y);
         if(maxn[y]+>maxn[x])
         {
             smaxn[x] = maxn[x];
             maxn[x] = maxn[y]+;
         }
         else if(maxn[y]+>smaxn[x])
             smaxn[x] = maxn[y]+;
     }
 }

 int main()
 {
     int T,Q;
     int i,j;
     int x,y,k;
     scanf("%d",&T);
     while(T--)
     {
         Init();
         scanf("%d%d",&n,&Q);
         for(i=; i<n; ++i)
         {
             scanf("%d %d",&x,&y);
             Add(x,y);
             Add(y,x);
         }
         DFS();
         int D=;  //树的直径
         for(i=; i<=n; ++i)
             D = MAX(D,maxn[i]+smaxn[i]);
         D++;
         while(Q--)
         {
             scanf("%d",&k);
             if(k<=D)
                 printf("%d\n",k-);
             else
                 printf("%d\n",D-+(k-D)*);
         }
     }
     return ;
 }