倍增LCA模板

//https://www.luogu.org/problemnew/show/P3379
#include<bits/stdc++.h>
#define maxn 500010
#define maxm 1000010
using namespace std;

struct edge{
    int to,next;
}e[maxm];

int point[maxn],n,m,rt,cnt;
int fa[maxn][],depth[maxn];

void addedge(int x,int y)
{
    e[++cnt].next=point[x];
    e[cnt].to=y;
    point[x]=cnt;
}

void dfs(int x,int from)
{
    depth[x]=depth[from]+;
    fa[x][]=from;
    for(int i=point[x];i;i=e[i].next)
    {
        int y=e[i].to;
        if(y==from)
            continue;
        dfs(y,x);
    }
}

int go_up(int x,int d)
{
    for(int j=;j>=;j--)
        if((<<j)&d)
            x=fa[x][j];
    return x;
}

int main()
{
    scanf("%d%d%d",&n,&m,&rt);
    for(int i=;i<=n-;i++)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        addedge(x,y);
        addedge(y,x);
    }
    dfs(rt,rt);
    for(int j=;j<=;j++)
        for(int i=;i<=n;i++)
            fa[i][j]=fa[fa[i][j-]][j-];
    for(int i=;i<=m;i++)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        if(depth[x]>depth[y])
            x=go_up(x,depth[x]-depth[y]);
        if(depth[y]>depth[x])
            y=go_up(y,depth[y]-depth[x]);
        for(int j=;j>=;j--)
            if(fa[x][j]!=fa[y][j])
                x=fa[x][j],y=fa[y][j];
        if(x!=y)
            x=fa[x][];
        printf("%d\n",x);
    }
    return ;
}