xdoj-1319 求树上任意一点的最大距离----利用树的直径

Posted 2020-12-27 xidian-mao

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 const int N=1e5+7;
 4 vector < vector <int> > g(N);
 5 int d1[N],d2[N],d[N];
 6 bool visit[N];
 7 struct node {
 8     int id;
 9     int w;
10 };
11 queue <node> q;
12 int n,m;
13 void dfs (int rt) {
14     memset (visit,0,sizeof(visit));
15     while (!q.empty()) q.pop();
16     node tmp={rt,0}; 
17     q.push(tmp); visit[tmp.id]=1; d[rt]=0;
18     while (!q.empty()) {
19         tmp=q.front(); q.pop();
20         for (int i=0;i<g[tmp.id].size();i++) {
21             int _next=g[tmp.id][i];
22             if (!visit[_next]) {
23                 node k={_next,tmp.w+1};
24                 q.push(k); visit[k.id]=1; d[_next]=k.w;
25             }
26         }
27     }
28     return ;
29 }
30 int find_max () {
31     int ans=0; int k;
32     for (int i=1;i<=n;i++) 
33         if (d[i]>ans) {
34             ans=d[i];
35             k=i;
36         }
37     return k;
38 }
39 void my_copy (int *a,int *b) {
40     for (int i=1;i<=n;i++)
41         a[i]=b[i];
42 }
43 int main ()
44 {
45     int T; scanf ("%d",&T);
46     while (T--) {
47         scanf ("%d",&n);
48         for (int i=1;i<=n;i++) g[i].clear();
49         for (int i=1;i<=n-1;i++) {
50             int u,v; scanf ("%d %d",&u,&v);
51             g[u].push_back(v);
52             g[v].push_back(u);
53         }
54         int x1,x2;
55         dfs (1); x1=find_max();
56         dfs (x1); x2=find_max();
57         my_copy (d1,d);
58         dfs (x2); 
59         my_copy (d2,d);
60         scanf ("%d",&m);
61         while (m--) {
62             int k,p; scanf ("%d %d",&k,&p);
63             int len=max (d1[k],d2[k]);
64             if (p<=len) printf("%d
",p+1);
65             else        printf("%d
",min (n,len+(p-len)/2+1) );
66         }
67     }
68     return 0;
69 }

感谢金牌果善意的提醒~~~树形dp还是有点麻烦。。