树上倍增求解LCA 模板

//详解见https://www.cnblogs.com/zwfymqz/p/7795299.html

//博主： 自为风月马前卒 https://www.cnblogs.com/zwfymqz/

// 洛谷 P3379 最近公共祖先 链接https://www.luogu.org/problem/P3379

#include<cstdio>
#include<cstring>
#include<iostream>
#include<string>
#include<algorithm>
using namespace std;

const int MAXN = 5e5+10;

struct node {// 链式向前星  具体原理见博客https://blog.csdn.net/acdreamers/article/details/16902023

int v, next; 12 } edge[2*MAXN]; 13 14 int head[2*MAXN];

int num = 1;
inline void add_edge(int x, int y) {
    edge[num].v = y;
    edge[num].next = head[x];
    head[x] = num++;
}

int f[MAXN][21];
int deep[MAXN];
int n, m, root;
void dfs(int cur) {
    for(int i = head[cur]; i != -1; i = edge[i].next) {
        if(!deep[edge[i].v]) {
            deep[edge[i].v] = deep[cur] + 1;
            f[edge[i].v][0] = cur;
            dfs(edge[i].v);
        }
    }
}

void PRE() {
    for(int j = 1; j <= 19; ++j) {
        for(int i = 1; i <= n; ++i) {
            f[i][j] = f[f[i][j-1]][j-1];
        }
    }
}

int LCA(int x, int y) {
    if(deep[x] < deep[y]) swap(x, y);
    for(int i = 19; i >= 0; --i) {
        if(deep[f[x][i]] >= deep[y]) {
            x = f[x][i];
        }
    }
    if(x == y) return x;
    for(int i = 19; i >= 0; --i) {
        if(f[x][i] != f[y][i]) {
            x = f[x][i];
            y = f[y][i];
        }
    }
    return f[x][0];
}

int main() {
    memset(head, -1, sizeof(head));
    scanf("%d%d%d", &n, &m, &root);
    int x, y;
    for(int i = 0; i != n-1; ++i) {
        scanf("%d%d", &x, &y);
        add_edge(x, y);
        add_edge(y, x);
    }
    deep[root] = 1;
    printf("%d
", head[root]);
    dfs(root);
    PRE();
    for(int i = 0; i != n; ++i) {
        scanf("%d%d", &x, &y);
        printf("%d
", LCA(x, y));
    }
    return 0;
}