TLE悬关

#include <bits/stdc++.h>
using namespace std;
int n,m,f[500005],book[500005],deep[500005],fa;
vector<int> a[500005]; 
void dfs(int x,int s,int d)
{
	deep[x]=d;
	f[x]=s;
	book[x]=1;
	for(int i=0;i<a[x].size();i++)
		if(book[a[x][i]]==0)
			dfs(a[x][i],x,d+1);
	return ;
}
int lca(int x,int y)
{
	if(deep[x]<deep[y])
		swap(x,y);
	while(deep[x]!=deep[y])
		x=f[x];
	if(x==y)
		return x;
	while(x!=y)
		x=f[x],y=f[y];
	return x;
}
int main()
{
	cin>>n>>m>>fa;
	for(int i=1;i<n;i++)
	{
		int x,y;
		cin>>x>>y;
		a[x].push_back(y);
		a[y].push_back(x);
	}
	dfs(fa,0,1);
	for(int i=1;i<=m;i++)
	{
		int x,y;
		cin>>x>>y;
		cout<<lca(x,y)<<endl;
	}
	return 0;
}