tarjan求LCA

模版题 https://www.luogu.org/problemnew/show/P3379

讲解 https://www.bilibili.com/video/av41067872/?p=4&t=1591

#include <bits/stdc++.h>
using namespace std;
const int maxn = 5e5+5;
struct qnode{
    int x,y,lca;
}query[maxn];
int fa[maxn];
bool vis[maxn];
vector<int> G[maxn];
vector<int> Q[maxn];
void init(){
    for(int i=0; i<maxn; i++)fa[i]=i;
}
int find(int x){
    return fa[x]==x?x:fa[x]=find(fa[x]);
}
void tarjan(int u){
    vis[u] = true;
    for(auto qid:Q[u]){
        if(query[qid].x == u){
            if(vis[query[qid].y]){
                query[qid].lca = find(query[qid].y);
            }
        }
        else{
            if(vis[query[qid].x]){
                query[qid].lca = find(query[qid].x);
            }
        }
    }
    for(auto v:G[u]){
        if(vis[v]) continue;
        tarjan(v);
        fa[v] = u;
    }
}
int main(){
    init();
    int n, m, s;
    scanf("%d%d%d", &n, &m, &s);
    int x, y;
    for(int i=1; i<n; i++){
        scanf("%d%d", &x, &y);
        G[x].push_back(y);
        G[y].push_back(x);
    }
    for(int i=1; i<=m;i++){
        scanf("%d%d", &query[i].x, &query[i].y);
        Q[query[i].x].push_back(i);
        Q[query[i].y].push_back(i);
    }
    tarjan(s);
    for(int i=1;i<=m; i++){
        printf("%d
",query[i].lca);
    }
}