tarjan求lca ：并查集+dfs

//参考博客 https://www.cnblogs.com/jsawz/p/6723221.html
#include<bits/stdc++.h>
using namespace std;
#define maxn 420000
struct Query{int to,nxt,lca;}q[maxn];
struct Edge{int to,nxt;}edge[maxn<<1];
int n,m,p,x,y,tot_e,tot_q,head_q[maxn],head_e[maxn];
int fa[maxn],vis[maxn];
void init(){
    memset(head_e,-1,sizeof head_e);
    memset(head_q,-1,sizeof head_q);
    memset(fa,-1,sizeof fa);
    tot_q=tot_q=0;
}
void addedge(int u,int v){
    edge[tot_e].to=v;edge[tot_e].nxt=head_e[u];head_e[u]=tot_e++;
}
void addquery(int u,int v){
    q[tot_q].to=v;q[tot_q].nxt=head_q[u];head_q[u]=tot_q++;
}
int find(int x){return fa[x]==x?x:fa[x]=find(fa[x]);}
int dfs(int u){
    fa[u]=u;vis[u]=1;
    for(int i=head_e[u];i!=-1;i=edge[i].nxt){
        int v=edge[i].to;
        if(vis[v])continue;
        dfs(v);fa[v]=u;
    }
    for(int i=head_q[u];i!=-1;i=q[i].nxt){
        int v=q[i].to;
        if(vis[v]) q[i].lca=q[i^1].lca=find(v);
    }
}
int main(){
    init();
    cin>>n>>m>>p;
    for(int i=1;i<n;i++){
        cin>>x>>y;
        addedge(x,y);
        addedge(y,x);
    }
    for(int i=0;i<m;i++){
        cin>>x>>y;
        addquery(x,y);
        addquery(y,x);
    }
    dfs(p);
    for(int i=0;i<m;i++)
        printf("%d ",q[i<<1].lca);
}