#1#2#3AC, 其余RE

问是什么原因？？？

#include<bits/stdc++.h>
using namespace std;
#define int long long
constexpr int N = 5e6 + 1145;
constexpr int LM = 22;

vector<int> G[N];
int dep[N], fa[LM][N], root;

void dfs(int now, int father){
    dep[now] = dep[father] + 1;
    fa[now][0] = father;
    for(int i = 1; i <= 20; i ++)
        fa[now][i] = fa[fa[now][i - 1]][i - 1];
    int len = G[now].size();
    for(int i = 0; i < len; i ++){
        if(G[now][i] != father){
            dfs(G[now][i], now);
        }
    }
}

int lca(int x, int y){
    if(dep[x] > dep[y])swap(x, y);
    int wnt = dep[y] - dep[x];
    for(int i = 20; i >= 0; i --){
        if((1 << i) <= wnt){
            wnt -= (1 << i);
            y = fa[y][i];
        }
    }
    if(x == y)return x;
    for(int i = 20; i >= 0; i --){
        if(fa[y][i] != fa[x][i]){
            x = fa[x][i], y = fa[y][i];
        }
    }
    return fa[x][0];
}

signed main(){
    ios::sync_with_stdio(false);cin.tie(0);
    int n, m, x, y, a, b;
    cin >> n >> m >> root;
    for(int i = 1; i < n ; i++){
        cin >> x >> y;
        G[x].push_back(y), G[y].push_back(x);
    }
    dfs(root, 0);
    for(int i = 1; i <= m; i ++){
        cin >> a >> b;
        cout << lca(a, b) << endl;
    }
    return 0;
}