LCA板子

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
struct zzz {
    int t, nex;
}e[500010 << 1]; int head[500010], tot;
void add(int x, int y) {
    e[++tot].t = y;
    e[tot].nex = head[x];
    head[x] = tot;
}
int depth[500001], fa[500001][22], lg[500001];
void dfs(int now, int fath) {
    fa[now][0] = fath; depth[now] = depth[fath] + 1;
    for(int i = 1; i <= lg[depth[now]]; ++i)
        fa[now][i] = fa[fa[now][i-1]][i-1];
    for(int i = head[now]; i; i = e[i].nex)
        if(e[i].t != fath) dfs(e[i].t, now);
}
int LCA(int x, int y) {
    if(depth[x] < depth[y]) swap(x, y);
    while(depth[x] > depth[y])
        x = fa[x][lg[depth[x]-depth[y]] - 1];
    if(x == y) return x;
    for(int k = lg[depth[x]] - 1; k >= 0; --k)
        if(fa[x][k] != fa[y][k])
            x = fa[x][k], y = fa[y][k];
    return fa[x][0];
}
int main() {
    int n, m, s; scanf("%d%d%d", &n, &m, &s);
    for(int i = 1; i <= n-1; ++i) {
        int x, y; scanf("%d%d", &x, &y);
        add(x, y); add(y, x);
    }
    for(int i = 1; i <= n; ++i)
        lg[i] = lg[i-1] + (1 << lg[i-1] == i);
    dfs(s, 0);
    for(int i = 1; i <= m; ++i) {
        int x, y; scanf("%d%d",&x, &y);
        printf("%d\n", LCA(x, y));
    }
    return 0;
}