Unac100求助

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=5e5+110;
int d[N],jump[N][21];
vector<int> v[N];
void dfs(int x,int fa,int sum){
	jump[x][0]=fa;
	d[x]=sum;
	for(int i=0;i<v[x].size();i++) if(v[x][i]!=fa) dfs(v[x][i],x,sum+1);
}
int lca(int x,int y){
	if(d[x]<d[y]) swap(x,y);
	for(int i=20;i>=0;i--) if(d[jump[x][i]]>=d[y]) x=jump[x][i];
	if(x==y) return x;
	for(int i=20;i>=0;i--) if(jump[x][i]!=jump[y][i]) x=jump[x][i],y=jump[y][i];
	return jump[x][0];
}
signed main(){
	int n,t,s;
	cin>>n>>t>>s;
	for(int i=1;i<n;i++){
		int x,y;
		cin>>x>>y;
		v[y].push_back(x);
		v[x].push_back(y);
	}
	dfs(s,0,1);
	for(int i=1;i<=20;i++) for(int j=1;j<=n;j++) jump[j][i]=jump[jump[j][i-1]][i-1];
	while(t--){
	    int a,b;
	    cin>>a>>b;
	    cout<<lca(a,b)<<endl;
	}
	return 0;
}