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：

 1 #include "stdio.h"

 2 #include "string.h"

 3 

 4 #define N 110000

 5 

 6 struct node

 7 {

 8     int x,y;

 9     bool visit;

10     int next;

11 } edge[2*N];

12 int idx,head[N];

13 

14 inline int MAX(int a,int b)

15 {

16     return a>b?a:b;

17 }

18 void Init()

19 {

20     idx=0;

21     memset(head,-1,sizeof(head));

22 }

23 void Add(int x,int y)

24 {

25     edge[idx].x=x;

26     edge[idx].y=y;

27     edge[idx].visit=false;

28     edge[idx].next=head[x];

29     head[x]=idx++;

30 }

31 

32 int n;

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

34 

35 void DFS(int x)

36 {

37     int i,y;

38     maxn[x] = smaxn[x] = 0;

39     for(i=head[x]; i!=-1; i=edge[i].next)

40     {

41         y = edge[i].y;

42         if(edge[i].visit) continue;

43         edge[i].visit = edge[i^1].visit = true;

44         DFS(y);

45         if(maxn[y]+1>maxn[x])

46         {

47             smaxn[x] = maxn[x];

48             maxn[x] = maxn[y]+1;

49         }

50         else if(maxn[y]+1>smaxn[x])

51             smaxn[x] = maxn[y]+1;

52     }

53 }

54 

55 int main()

56 {

57     int T,Q;

58     int i,j;

59     int x,y,k;

60     scanf("%d",&T);

61     while(T--)

62     {

63         Init();

64         scanf("%d%d",&n,&Q);

65         for(i=1; i<n; ++i)

66         {

67             scanf("%d %d",&x,&y);

68             Add(x,y);

69             Add(y,x);

70         }

71         DFS(1);

72         int D=0;  //树的直径

73         for(i=1; i<=n; ++i)

74             D = MAX(D,maxn[i]+smaxn[i]);

75         D++;

76         while(Q--)

77         {

78             scanf("%d",&k);

79             if(k<=D)

80                 printf("%d\n",k-1);

81             else

82                 printf("%d\n",D-1+(k-D)*2);

83         }

84     }

85     return 0;

86 }