TLE三个点，大佬帮忙优化下

#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
vector<int>bian[500010];
vector<int>pos[500010];
bool ru[500010];
void dfs(int a)
{
	ru[a]=1;
	pos[a].push_back(a);
	int m=bian[a].size();
	for(int i=0;i<m;i++)
	{
		int k=bian[a][i];
		if(!ru[k])
		{
			pos[k]=pos[a];
			dfs(k);
		}
	}
	bian[a].clear();
}
int lca(int a,int b)
{
	int l=0,r=min(pos[a].size(),pos[b].size());
	while(l<r)
	{
		int mid=(l+r+1)>>1;
		if(pos[a][mid]==pos[b][mid])
		l=mid;
		else r=mid-1;
	}
	return pos[a][l];
}
int main()
{
	int a,b,c,d,e;
	cin>>a>>b>>c;
	for(int i=1;i<a;i++)
	{
		cin>>d>>e;
		bian[d].push_back(e);
		bian[e].push_back(d);
	}
	dfs(c);
	while(b--)
	{
		cin>>d>>e;
		cout<<lca(d,e)<<"\n";
	}
}