过了样例，但是提交输出全0求调

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pll;
const int N = 500010;
int n, m, k;
int h[N], e[N], ne[N], idx;
int depth[N], fa[N][21];
void add(int a, int b) {
	e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}
void bfs() {
	queue<int> q;
	q.push(k);
	depth[k] = 1;
	while (q.size()) {
		int t = q.front();
		q.pop();
		for (int i = h[t] ; ~i ; i = ne[i]) {
			int j = e[i];
			depth[j] = depth[t] + 1;
			fa[j][0] = t;
			q.push(j);
			for (int p = 1 ; p <= 18 ; ++p) {
				fa[j][p] = fa[fa[j][p - 1]][p - 1];
			}
		}
	}
}
int lca(int a, int b) {
	if (depth[a] < depth[b]) swap(a, b);
	for (int i = 20 ; i >= 0 ; --i) {
		if (depth[fa[a][i]] >= depth[b]) {
			a = fa[a][i];
		}
	}
	if (a == b) return a;
	for (int i = 20 ; i >= 0 ; --i) {
		if (fa[a][i] != fa[b][i]) {
			a = fa[a][i];
			b = fa[b][i];
		}
	}
	return fa[a][0];
}
int main() {
	cin >> n >> m >> k;
	memset(h, -1, sizeof h);
	for (int i = 1 ; i < n ; ++i) {
		int a, b;
		cin >> a >> b;
		if (a != b) add(b, a);
	}
	bfs();
	while (m -- ) {
		int a, b;
		cin >> a >> b;
		cout << lca(a, b) << endl;
	}
	return 0;
}