LCA求调

#include<bits/stdc++.h>
using namespace std;
int n,q,root,k;
int st[500030][20],deep[500030];
vector<int> v[500030];
void count_deep(int x,int father){
	deep[x]=deep[father]+1;
	st[x][0]=father;
	for(int i=1;(1<<i)<=deep[father];i++){
		st[x][i]=st[st[x][i-1]][i-1];
	}
	for(int i=0;i<v[x].size();i++){
		count_deep(v[x][i],x);
	}
	
}
int lca(int a,int b){
	int x=deep[a],y=deep[b];
	if(x!=y){
		if(x<y){
			swap(a,b);
			swap(deep[a],deep[b]);
		}
		for(int i=0;i<=k;i++){
			if((1<<i)!=(a-b)){
				a=st[a][i];
			}
		}
	}
	if(a==b) return a;
	for(int i=k;i>=0;i--){
		if(deep[st[a][i]]==0) continue;
		if(st[a][i]==st[b][i]) continue;
		a=st[a][i],b=st[b][i];
	}
	return st[a][0];
}
int main(){
	cin>>n>>q>>root;
	k=log(n)/log(2)+0.5;
	for(int i=1;i<n;i++){
		int a,b;
		cin>>a>>b;
		v[b].push_back(a);
	}
	count_deep(root,0);
	while(q--){
		int a,b;
		cin>>a>>b;
		int x=lca(a,b);
		cout<<x<<endl;
	}
}