原本可以A，再交就一直TLE。

蒟蒻的记录

代码：

#include<iostream>
#include<algorithm>
#include<cstring>
#include<climits>
#include<cmath>
#include<queue>
#include<vector>
#include<stack>
#include<tuple>
#include<unordered_map>
#include<map>
#include<unordered_set>
#include<set>
#define rep(i,a,b,c) for(int i=(a);i<=(b);i+=(c))
#define per(i,a,b,c) for(int i=(a);i>=(b);i-=(c))
#define ALL(a) (a).begin(),(a).end()
using namespace std;
using ll = long long;
const int INF = 0x3f3f3f3f;
const int N = 5e5 + 5;
const int K = 32;

vector<int> g[N];
int n, m, s;
int f[N][K + 5], depth[N];

inline int read()
{
    int x=0,f=1;
    char ch=getchar();
    while(ch<'0'||ch>'9')
    {
        if(ch=='-')
            f=-1;
        ch=getchar();
    }
    while(ch>='0' && ch<='9')
        x=x*10+ch-'0',ch=getchar();
    return x*f;
}

namespace LCA {
    void dfs(int u, int fa) {
        //cout << f[u][0] << " ----> " << u << endl;
        f[u][0] = fa;
        depth[u] = depth[fa] + 1;
        rep(k, 1, K, 1) {
            int mid = f[u][k - 1];
            f[u][k] = f[mid][k - 1];
        }
        for (auto v : g[u]) {
            if (depth[v] > depth[u] + 1) {
                dfs(v, u);
            }
        }

    }

    int lca(int a, int b) {
        if (depth[a] > depth[b]) swap(a, b);
        per(k, K, 0, 1) {
            int pb = f[b][k];
            if (depth[pb] >= depth[a]) {
                b = pb;
            }
        }
        if (a == b) return a;
        per(k, K, 0, 1) {
            int pa = f[a][k], pb = f[b][k];
            if (pb != pa) {
                a = pa, b = pb;
            }
        }
        return f[a][0];
    }

    void init() {
        memset(f, 0, sizeof(f));
        memset(depth, 0x3f, sizeof(depth));
        depth[0] = 0;
        LCA::dfs(s, 0);
    }
}

int main() {
    n = read();
    m = read();
    s = read();
    rep(i, 1, n - 1, 1) {
        int u = read(), v = read();
        g[u].push_back(v);
        g[v].push_back(u);
    }
    LCA::init();
    //rep(u, 1, n, 1)cout << u << " -> " << f[u][0] << endl;
    rep(i, 1, m, 1) {
        int u = read(), v = read();
        cout << LCA::lca(u, v) << endl;
    }
    return 0;
}

球条，谢谢