【重链剖分】洛谷 P3379 【模板】最近公共祖先（LCA）

题链

#include <bits/stdc++.h>
using namespace std;
#define LL long long
#define ll long long
#define ULL unsigned long long
#define Pair pair<LL,LL>
#define ls rt<<1
#define rs rt<<1|1
#define Pi acos(-1.0)
#define eps 1e-6
#define DBINF 1e100
#define mod 998244353
#define MAXN 1e18
#define MS 500009

int n,m,rt;
vector<int > vc[MS]; 
int fa[MS];
int sz[MS];
int zson[MS];
int dep[MS];

int tim;
int top[MS];
int dfn[MS];

void dfs1(int u,int f){ // dfs1 => fa[] ,sz[] ,zson[] ,dep[]
	fa[u] = f;
	sz[u] = 1;
	dep[u] = dep[f]+1;
	zson[u] = 0;
	int maxn_zson = 0;
	for(auto &v:vc[u]){
		if(v == f) continue;
		dfs1(v,u);
		sz[u] += sz[v];
		if(sz[v] > maxn_zson){
			zson[u] = v;
			maxn_zson = sz[v];
		}
	}
}

void dfs2(int u,int tp){ // dfs2 => tim ,top[] ,dfn[]
	dfn[u] = ++tim;
	top[u] = tp;
	if(zson[u]) dfs2(zson[u],tp);
	for(auto &v:vc[u]){
		if(v != fa[u] && v != zson[u]){
			dfs2(v,v);
		}
	}
}

int cal(int u,int v){
	while(top[u] != top[v]){
		if(dep[ top[u] ] < dep[ top[v] ]) swap(u,v); // ***选择 *链头深度大 的向上跳 
		u = fa[top[u]];
	}
	if(dep[u] < dep[v]) return u; // 深度小的为 LCA 
	return v;
}

int main() {
	ios::sync_with_stdio(false);
	cin >> n >> m >> rt;
	for(int i=1;i<=n-1;i++){
		int u,v;
		cin >> u >> v;
		vc[u].push_back(v);
		vc[v].push_back(u);
	}
	dfs1(rt,rt);
	dfs2(rt,rt);
	while(m--){
		int u,v;
		cin >> u >> v;
		cout << cal(u,v) << "\n";
	}
	
	
	return 0;
}