RT,已开O2,懒得写链表

#include <iostream>
#include <vector>
#include <cstdio>
#pragma GCC diagnostic error "-std=c++11"
#pragma GCC target("avx")
#pragma GCC optimize(3)
#pragma GCC optimize("Ofast")
#pragma GCC optimize("inline")
#pragma GCC optimize("-fgcse")
#pragma GCC optimize("-fgcse-lm")
#pragma GCC optimize("-fipa-sra")
#pragma GCC optimize("-ftree-pre")
#pragma GCC optimize("-ftree-vrp")
#pragma GCC optimize("-fpeephole2")
#pragma GCC optimize("-ffast-math")
#pragma GCC optimize("-fsched-spec")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("-falign-jumps")
#pragma GCC optimize("-falign-loops")
#pragma GCC optimize("-falign-labels")
#pragma GCC optimize("-fdevirtualize")
#pragma GCC optimize("-fcaller-saves")
#pragma GCC optimize("-fcrossjumping")
#pragma GCC optimize("-fthread-jumps")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-fwhole-program")
#pragma GCC optimize("-freorder-blocks")
#pragma GCC optimize("-fschedule-insns")
#pragma GCC optimize("inline-functions")
#pragma GCC optimize("-ftree-tail-merge")
#pragma GCC optimize("-fschedule-insns2")
#pragma GCC optimize("-fstrict-aliasing")
#pragma GCC optimize("-fstrict-overflow")
#pragma GCC optimize("-falign-functions")
#pragma GCC optimize("-fcse-skip-blocks")
#pragma GCC optimize("-fcse-follow-jumps")
#pragma GCC optimize("-fsched-interblock")
#pragma GCC optimize("-fpartial-inlining")
#pragma GCC optimize("no-stack-protector")
#pragma GCC optimize("-freorder-functions")
#pragma GCC optimize("-findirect-inlining")
#pragma GCC optimize("-fhoist-adjacent-loads")
#pragma GCC optimize("-frerun-cse-after-loop")
#pragma GCC optimize("inline-small-functions")
#pragma GCC optimize("-finline-small-functions")
#pragma GCC optimize("-ftree-switch-conversion")
#pragma GCC optimize("-foptimize-sibling-calls")
#pragma GCC optimize("-fexpensive-optimizations")
#pragma GCC optimize("-funsafe-loop-optimizations")
#pragma GCC optimize("inline-functions-called-once")
#pragma GCC optimize("-fdelete-null-pointer-checks")
using namespace std;

const int MAXN = 500010;

vector<int> G[MAXN];
int root, depth[MAXN], parent[21][MAXN], n, m;

void dfs(int v, int p, int d) {
    depth[v] = d;
    parent[0][v] = p;
    for (register int i=0; i<G[v].size(); i++) {
        if (G[v][i] != p) dfs(G[v][i], v, d+1);
    }
}

void init() {
    dfs(root, -1, 1);
    for (register int k=0; k+1<=20; k++) 
        for (register int v=1; v<=n; v++) {
            if (parent[k][v] < 0) parent[k+1][v] = -1;
            else parent[k+1][v] = parent[k][parent[k][v]];
        }
}

inline int lca(int u, int v) {
    if (depth[u] > depth[v]) swap(u, v);
    for (register int k=0; k<=20; k++)
        if ((depth[v]-depth[u]) >> k & 1) v = parent[k][v];
    if (u == v) return u;
    for (register int k=20; k>=0; k--)
        if (parent[k][v] != parent[k][u]) {
            v = parent[k][v];
            u = parent[k][u];
        }
    return parent[0][u];
}

inline int read()
{
    int ans=0;
    char ch=getchar();
    while (ch<'0' || ch>'9') ch=getchar();
    while (ch>='0' && ch<='9')ans=ans*10+ch-'0',ch=getchar();
    return ans;
}
inline void write(int x)
{
    char f[200];
    int tmp=x>0?x:-x,cnt=0;
    if(x<0)putchar('-');
    while(tmp>0)f[cnt++]=tmp%10+'0',tmp/=10;
    while(cnt>0)putchar(f[--cnt]);
    putchar('\n');
}

void addedge(int x, int y) {
    G[x].push_back(y);
    G[y].push_back(x);
}

int main() {
    //ios::sync_with_stdio(false);
    n = read(), m = read(), root = read();
    for (register int i=1; i<=n-1; i++) {
        int x = read(), y = read();
        addedge(x, y);
    }
    init();
    for (register int i=1; i<=m; i++) {
        int x = read(), y = read();
        write(lca(x, y));
    }
    return 0;
}