下载N≤500000，真心调不出来。

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<vector>
#include<queue>
using namespace std;
const int N = 1e6+10;
int head[N],next[N],edge[N],cnt = 0;
int dep[N],f[500000 + 10][17],lg[N];
void add(int x,int y) {
	next[++cnt] = head[x];
   edge[cnt] = y;
   head[x] = cnt;
}
void addd(int x,int y) {add(x,y);add(y,x);}
void dfs(int fa,int now) {
    f[now][0] = fa;dep[now] = dep[fa]+1;
    for(int i = 1; i <= lg[now]; i++)f[now][i] = f[f[now][i-1]][i-1];
    for(int i = head[now]; i; i = next[i])
        if(edge[i] != fa)dfs(now,edge[i]);
}
int LCA(int x,int y) {
    if(dep[x] < dep[y])swap(x,y);
    for(int i = lg[dep[x]]; i >= 0; i--) {
        if(dep[f[x][i]] >= dep[y])x = f[x][i];
    	if(x == y)return x;        
    }
	for(int i = lg[dep[x]]-1; i >= 0; i--) {		
		if(f[x][i] != f[y][i]){
			x = f[x][i], y = f[y][i]; 
		}
	}return f[x][0];
}
int main() {
	int x,y,n,m,s;scanf("%d%d%d",&n,&m,&s);
	for(int i = 1; i <= n; ++i)
		lg[i] = lg[i-1] + (1 << lg[i-1] == i);
    for(int i = 1; i <= n-1; i++)scanf("%d%d",&x,&y),addd(x,y);
    dfs(0,s);
    for(int i = 1; i <= m; i++) {
        scanf("%d%d",&x,&y);
        int ans = LCA(x,y);
        printf("%d\n",ans);
    }
    return 0;
}