模板_LCA

// luogu-judger-enable-o2
#include<bits/stdc++.h>
#define maxn 1000002
//#define int long long
//#define kcz
using namespace std;
inline int read()
{
    char x = getchar();
    int lin = 0,f = 1;
    while(x < '0' || x > '9')
    {
        if(x == '-') f = -1;
        x = getchar();
    }
    while(x >= '0' && x <= '9')
    {
        lin = (lin << 1) + (lin << 3) + x - '0';
        x = getchar();
    }
    return lin * f;
}
#define PII pair<int,int>
#define fir first
#define sec second
#define ma(a,b) make_pair(a,b)
#define inf 123123123
int head[maxn],u,v,tot,n,m,dep[maxn],q,fa[maxn][23],rt;
struct st{
    int v,nex;
}s[maxn];
void add(int u,int v)
{
    s[++tot] = (st) { v,head[u] };
    head[u] = tot;
}
void dfs(int now,int F)
{
    fa[now][0] = F;dep[now]=dep[F]+1;
    for(int i=1;i<=22;i++)fa[now][i]=fa[fa[now][i-1]][i-1];
    for(int i = head[now]; i; i = s[i].nex)
        if(s[i].v != F)
        {
            dfs(s[i].v,now);
        }
}
int lca(int a,int b)
{
    if(dep[a] > dep[b]) swap(a,b);
    int len = dep[b] - dep[a];
    for(int i = 0; (1 << i) <= len; i++)
        if(len & (1 << i))
            b = fa[b][i];
    if(a != b)
    {
        for(int i = 22; i >= 0; i--)
        {
            if(fa[a][i] != fa[b][i])
            {
                a = fa[a][i];
                b = fa[a][i];
            }
        }
        a = fa[a][0];
    }
    return a;
}
signed main(){
    n = read(); q = read(); rt = read();
    for(int i = 1; i < n; i++)
    {
        u = read(); v = read();
        add(u,v); add(v,u);
    }
    dfs(rt,0);
    for(int i = 1; i <= q; i++)
    {
        u = read(); v = read();
        printf("%d
",lca(u,v));
    }
}