树剖lca（模板）

#include<cstdio>
#include<algorithm>
#define fo(i,a,b) for (int (i)=(a);(i)<=(b);(i)++)
#define Fo(i,x) for (int i=head[x];i;i=next[i])
using namespace std;
const int N=500000+255;
int f[N],top[N],son[N],head[N],to[N*2],next[N*2],size[N],d[N];
int n,m,s,tot,x,y;
void add(int x,int y){
	to[++tot]=y;
	next[tot]=head[x];
	head[x]=tot;
}
void dfs(int x){
	size[x]=1;
	Fo(i,x) {
		int v=to[i];
		if (v==f[x]) continue;
		d[v]=d[x]+1;
		f[v]=x;
		dfs(v);
		size[x]+=size[v];
		if (size[v]>size[son[x]]) son[x]=v;
	}
}
void Dfs(int x,int T){
	top[x]=T;
	if (!son[x]) return;
	Dfs(son[x],T);
	Fo(i,x){
		int v=to[i];
		if (v==f[x]||(v==son[x])) continue;
		Dfs(v,v);
	}
}
int lca(int x,int y){
	while (top[x]!=top[y]){
		if (d[top[x]]>d[top[y]]) x=f[top[x]];
		else y=f[top[y]];
	}
	return d[x]<d[y]?x:y;
}
int main(){
	//freopen("a.in","r",stdin);
	//freopen("a.out","w",stdout);
	scanf("%d%d%d",&n,&m,&s);
	fo(i,1,n-1){
		scanf("%d%d",&x,&y);
		add(x,y);add(y,x);
	}
	dfs(s);
	Dfs(s,s);
	//fo(i,1,n) printf("%d ",top[i]);
	//return 0;
	while (m--){
		scanf("%d%d",&x,&y);
		printf("%d\n",lca(x,y));
	}
	return 0;
}