红100求调

#include<bits/stdc++.h>
using namespace std;
int n,m,s,f[1000001][30],b[31]={1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194394,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824};
struct node
{
	int isvisit,depth;
	vector<int> neighbour;
}a[500001];
void start(int x)
{
	for(int i=1;i<=n;i++)
	{
		a[i].depth=0;
		a[i].isvisit=0;
	}
	a[x].isvisit=1;
	return;
}
void dfs(int x,int fa)
{
	f[x][0]=fa;
	a[x].depth=a[fa].depth+1;
	for(int i=0;i<a[x].neighbour.size();i++)
	{
		if(a[a[x].neighbour[i]].isvisit==0)
		{
			a[a[x].neighbour[i]].isvisit=1;
			dfs(a[x].neighbour[i],x);
		}
	}
	return;
}
void init()
{
	for(int j=1;b[j]<=n;j++)
	{
		for(int i=1;i<=n;i++)
		{
			f[i][j]=f[f[i][j-1]][j-1];
		}
	}
}
int lca(int x,int y)
{
	if(a[x].depth<a[y].depth)
	{
		swap(x,y);
	}
	for(int i=22;i>=0;i--)
	{
		if(a[f[x][i]].depth>=a[y].depth)
		{
			x=f[x][i];
		}
	}
	if(x==y)
	{
		return x;
	}
	for(int i=22;i>=0;i--)
	{
		if(f[x][i]!=f[y][i])
		{
			x=f[x][i];
			y=f[y][i];
		}
	}
	return f[x][0];
}
int main()
{
	cin>>n>>m>>s;
	for(int i=1;i<n;i++)
	{
		int x,y;
		cin>>x>>y;
		a[x].neighbour.push_back(y);
		a[y].neighbour.push_back(x); 
	}
	start(s);
	dfs(s,0);
	init();
	while(m--)
	{
		int x,y;
		cin>>x>>y;
		cout<<lca(x,y)<<endl;
	}
 	return 0;
}