[luogu-3379]【模板】最近公共祖先（LCA） 题解

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#define rep(i,a,n) for (int i=a;i<=n;i++)
#define per(i,a,n) for (int i=a;i>=n;i--)
#define Clear(a,x) memset(a,x,sizeof(a))
#define ll long long
#define INF 2000000000
#define eps 1e-8
using namespace std;
int read(){
int x=0,f=1;
char ch=getchar();
while (ch<'0'||ch>'9') f=ch=='-'?-1:f,ch=getchar();
while (ch>='0'&&ch<='9') x=x*10+ch-'0',ch=getchar();
return x*f;
}
const int maxn=500005;
int ans[maxn],anc[maxn],f[maxn],flag[maxn];
int n,m,root;
struct node{
int vet[maxn<<1],next[maxn<<1],head[maxn],road[maxn<<1];
int len;
void add(int u,int v,int w){
vet[++len]=v;
next[len]=head[u];
head[u]=len;
road[len]=w;
}
}edge,edge_q;
int find(int k){
return f[k]==k?k:f[k]=find(f[k]);
}
void Union(int x,int y){
int xx=find(x),yy=find(y);
f[xx]=yy;
}
void dfs(int u){
anc[find(u)]=u;
flag[u]=1;
for (int e=edge.head[u];e;e=edge.next[e]){
int v=edge.vet[e];
if (flag[v]) continue;
dfs(v);
Union(u,v);
anc[find(u)]=u;
}
flag[u]=2;
for (int e=edge_q.head[u];e;e=edge_q.next[e]){
int v=edge_q.vet[e];
int i=edge_q.road[e];
if (flag[v]==2){
int x=anc[find(v)];
ans[i]=x;
}
}
}
int main(){
n=read(),m=read(),root=read();
rep(i,1,n-1){
int u=read(),v=read();
edge.add(u,v,0),edge.add(v,u,0);
}
rep(i,1,m){
int u=read(),v=read();
edge_q.add(u,v,i),edge_q.add(v,u,i);
}
rep(i,1,n) f[i]=i;
dfs(root);
rep(i,1,m) printf("%d\n",ans[i]);
return 0;
}