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。问你从随意点为起点。走k（k<=n)个点，至少须要走多少距离（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；

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

#define N 110000

struct node
{
    int x,y;
    bool visit;
    int next;
} edge[2*N];
int idx,head[N];

inline int MAX(int a,int b)
{
    return a>b?a:b;
}
void Init()
{
    idx=0;
    memset(head,-1,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] = 0;
    for(i=head[x]; i!=-1; i=edge[i].next)
    {
        y = edge[i].y;
        if(edge[i].visit) continue;
        edge[i].visit = edge[i^1].visit = true;
        DFS(y);
        if(maxn[y]+1>maxn[x])
        {
            smaxn[x] = maxn[x];
            maxn[x] = maxn[y]+1;
        }
        else if(maxn[y]+1>smaxn[x])
            smaxn[x] = maxn[y]+1;
    }
}

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=1; i<n; ++i)
        {
            scanf("%d %d",&x,&y);
            Add(x,y);
            Add(y,x);
        }
        DFS(1);
        int D=0;  //树的直径
        for(i=1; i<=n; ++i)
            D = MAX(D,maxn[i]+smaxn[i]);
        D++;
        while(Q--)
        {
            scanf("%d",&k);
            if(k<=D)
                printf("%d
",k-1);
            else
                printf("%d
",D-1+(k-D)*2);
        }
    }
    return 0;
}