LCA 树链剖分

//LCA
//树链剖分 在线 
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;
int n,m,root,cnt,head[500001];
int hvyson[500001],fa[500001],siz[500001],dep[500001],top[500001];
struct uio{
    int to,next;
}edge[1000001];
void add(int x,int y)
{
    edge[++cnt].next=head[x];
    edge[cnt].to=y;
    head[x]=cnt;
}
void dfs1(int x,int f,int depth)
{
    dep[x]=depth;
    fa[x]=f;
    siz[x]=1;
    int maxson=-1;
    for(int i=head[x];i;i=edge[i].next)
    {
        int y=edge[i].to;
        if(y==f)
            continue;
        dfs1(y,x,depth+1);
        siz[x]+=siz[y];
        if(siz[y]>maxson)
        {
            hvyson[x]=y;
            maxson=siz[y];
        }
    }
}
void dfs2(int x,int topf)
{
    top[x]=topf;
    if(!hvyson[x])
        return;
    dfs2(hvyson[x],topf);
    for(int i=head[x];i;i=edge[i].next)
    {
        int y=edge[i].to;
        if(y==fa[x]||y==hvyson[x])
            continue;
        dfs2(y,y);
    }
}
int lca(int x,int y)
{
    while(top[x]!=top[y])
    {
        if(dep[top[x]]<dep[top[y]])
            swap(x,y);
        x=fa[top[x]];
    }
    return dep[x]<dep[y]? x : y;
}
void do_something()
{
    for(int i=1;i<=m;i++)
    {
        int u,v;
        scanf("%d%d",&u,&v);
        printf("%d\n",lca(u,v));
    }
}
int main()
{
    scanf("%d%d%d",&n,&m,&root);
    for(int i=1;i<n;i++)
    {
        int u,v;
        scanf("%d%d",&u,&v);
        add(u,v);
        add(v,u);
    }
    dfs1(root,0,1);
    dfs2(root,root);
    do_something();
     return 0;
}