LCA

#include <iostream>
#include <cstdio>
#define Max 500010

using namespace std;

void read(int &x)
{
    x=0;int f=1;
    char ch=getchar();
    while(ch>'9'||ch<'0')
    {
        if(ch=='-') f=-1;
        ch=getchar();
    }
    while(ch>='0'&&ch<='9')
    {
        x=x*10+(int)ch-48;
        ch=getchar();
    }
    x*=f;
}
struct Point
{
    int fa;
    int deep;
    int chain;
    int size;
}point[Max];
struct Edge
{
    int from,to,next;
}edge[Max<<1];
int head[Max];
int tot;
int count;
inline void add(int from,int to)
{
    tot++;
    edge[tot].from=from;
    edge[tot].to=to;
    edge[tot].next=head[from];
    head[from]=tot;
}
void dfs1(int now,int father)
{
    int pos=count++;
    point[now].fa=father;
    point[now].deep=point[point[now].fa].deep+1;
    for(int i=head[now];i;i=edge[i].next)
    {
        if(edge[i].to==father)
        continue;
        dfs1(edge[i].to,now);
    }
    point[now].size=count-pos;
}
void dfs2(int now,int chain)
{
    int pos=0;
    point[now].chain=chain;
    for(int i=head[now];i;i=edge[i].next)
    {
        if(point[edge[i].to].chain)
        continue;
        if(point[pos].size<point[edge[i].to].size)
        pos=edge[i].to;
    }
    if(pos) dfs2(pos,chain);
    else return;
    for(int i=head[now];i;i=edge[i].next)
    {
        if(edge[i].to==pos||edge[i].to==point[now].fa)
        continue;
        dfs2(edge[i].to,edge[i].to);
    }
}
int LCA(int x,int y)
{
    while(point[x].chain!=point[y].chain)
    {
        if(point[point[x].chain].deep<point[point[y].chain].deep)
        swap(x,y);
        x=point[point[x].chain].fa;
    }
    return point[x].deep<point[y].deep?x:y;
}
int Root,M,N,x,y;
int main()
{
    read(N);
    read(M);
    read(Root);
    for(int i=1;i<N;++i)
    {
        read(x);
        read(y);
        add(x,y);
        add(y,x);
    }
    dfs1(Root,0);
    count=0;
    dfs2(Root,Root);
    while(M--)
    {
        read(x);read(y);
        printf("%d
",LCA(x,y));
    }
    return 0;
}