题意：
给你一棵树，树上有些点是有人的，问你选一个点，最短的（最远的那个人的距离）是多少。
思路：
其实就是树的直径，两遍dfs，dfs第二遍的时候遇到人就更新直径就行了，ans是/2，奇数的话+1。
1 #define IOS ios_base::sync_with_stdio(0); cin.tie(0); 2 #include <cstdio>//sprintf islower isupper 3 #include <cstdlib>//malloc exit strcat itoa system("cls") 4 #include <iostream>//pair 5 #include <fstream> 6 #include <bitset> 7 //#include <map> 8 //#include<unordered_map> 9 #include <vector> 10 #include <stack> 11 #include <set> 12 #include <string.h>//strstr substr 13 #include <string> 14 #include <time.h>//srand(((unsigned)time(NULL))); Seed n=rand()%10 - 0~9; 15 #include <cmath> 16 #include <deque> 17 #include <queue>//priority_queue<int, vector<int>, greater<int> > q;//less 18 #include <vector>//emplace_back 19 //#include <math.h> 20 //#include <windows.h>//reverse(a,a+len);// ~ ! ~ ! floor 21 #include <algorithm>//sort + unique : sz=unique(b+1,b+n+1)-(b+1);+nth_element(first, nth, last, compare) 22 using namespace std;//next_permutation(a+1,a+1+n);//prev_permutation 23 #define fo(a,b,c) for(register int a=b;a<=c;++a) 24 #define fr(a,b,c) for(register int a=b;a>=c;--a) 25 #define mem(a,b) memset(a,b,sizeof(a)) 26 #define pr printf 27 #define sc scanf 28 void swapp(int &a,int &b); 29 double fabss(double a); 30 int maxx(int a,int b); 31 int minn(int a,int b); 32 int Del_bit_1(int n); 33 int lowbit(int n); 34 int abss(int a); 35 //const long long INF=(1LL<<60); 36 const double E=2.718281828; 37 const double PI=acos(-1.0); 38 const int inf=(1<<29); 39 const double ESP=1e-9; 40 const int mod=(int)1e9+7; 41 const int N=(int)1e5+10; 42 43 vector<vector<int> >v(N); 44 bool is[N],vis[N]; 45 int pre1,pre2; 46 int max_=-1; 47 48 void dfs1(int u,int depth) 49 { 50 if(is[u]) 51 { 52 if(max_<depth) 53 pre2=u,max_=depth; 54 } 55 vis[u]=1; 56 int sz=v[u].size(); 57 fo(i,0,sz-1) 58 { 59 int to=v[u][i]; 60 if(!vis[to]) 61 dfs1(to,depth+1); 62 } 63 } 64 void dfs2(int u,int depth) 65 { 66 vis[u]=1; 67 if(is[u]) 68 max_=maxx(max_,depth); 69 int sz=v[u].size(); 70 fo(i,0,sz-1) 71 { 72 int to=v[u][i]; 73 if(!vis[to]) 74 dfs1(to,depth+1); 75 } 76 } 77 78 int main() 79 { 80 int n,k; 81 sc("%d%d",&n,&k); 82 fo(i,1,n-1) 83 { 84 int a,b; 85 sc("%d%d",&a,&b); 86 v[a].push_back(b); 87 v[b].push_back(a); 88 } 89 fo(i,1,k) 90 { 91 int a; 92 sc("%d",&a); 93 is[a]=1; 94 pre1=a; 95 } 96 dfs1(pre1,1); 97 mem(vis,0); 98 max_=-1; 99 dfs2(pre2,0); 100 pr("%d ",(max_+1)/2); 101 return 0; 102 } 103 104 /**************************************************************************************/ 105 106 int maxx(int a,int b) 107 { 108 return a>b?a:b; 109 } 110 111 void swapp(int &a,int &b) 112 { 113 a^=b^=a^=b; 114 } 115 116 int lowbit(int n) 117 { 118 return n&(-n); 119 } 120 121 int Del_bit_1(int n) 122 { 123 return n&(n-1); 124 } 125 126 int abss(int a) 127 { 128 return a>0?a:-a; 129 } 130 131 double fabss(double a) 132 { 133 return a>0?a:-a; 134 } 135 136 int minn(int a,int b) 137 { 138 return a<b?a:b; 139 }