倍增法求LCA

 1 #include<cstdio>
 2 #include<iostream>
 3 #define N 42000
 4 using namespace std;
 5 int next[N],to[N],head[N],num,deep[N],father[N][21],n,m,p,a,b,c;
 6 void add(int false_from,int false_to){
 7     next[++num]=head[false_from];
 8     to[num]=false_to;
 9     head[false_from]=num;
10 }
11 void dfs(int x){
12     deep[x]=deep[father[x][0]]+1;
13     for(int i=0;father[x][i];i++)
14         father[x][i+1]=father[father[x][i]][i];
15     for(int i=head[x];i;i=next[i])
16         if(!deep[to[i]]){
17             father[to[i]][0]=x;
18             dfs(to[i]);
19         }
20 }
21 int lca(int x,int y){
22     if(deep[x]>deep[y])
23         swap(x,y);
24     for(int i=20;i>=0;i--)
25         if(deep[father[y][i]]>=deep[x])
26             y=father[y][i];
27     if(x==y)
28         return y;
29     for(int i=20;i>=0;i--)
30         if(father[y][i]!=father[x][i]){
31             y=father[y][i];
32             x=father[x][i];
33         }
34     return father[x][0];
35 }
36 int main(){
37     scanf("%d%d%d",&n,&m,&p);
38     for(int i=1;i<n;++i){
39         scanf("%d%d",&a,&b);
40         add(a,b);
41         add(b,a);
42     }
43     dfs(p);
44     for(int i=1;i<=m;++i){
45         scanf("%d%d",&a,&b);
46         printf("%d ",lca(a,b));
47     }
48     return 0;
49 }