LCA+tarjan

#include<cstring>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<ctime>
using namespace std;
using namespace std;
const int N=500005;
int cnt,fa[N],ans[N],head[N],rhead[N],dis[N];
bool vis[N];
struct node{
    int u,v,next,id;
}edge[N],redge[N];
int find(int x){
    if(fa[x]==-1){
        return x;
    }
    return fa[x]=find(fa[x]);
}
void Union(int a,int b){
    if(find(a)!=find(b)){
        fa[find(b)]=find(a);
    }
}
void push1(int u,int v){
    edge[cnt].u=u;
    edge[cnt].v=v;
    edge[cnt].next=head[u];
    head[u]=cnt;
    cnt++;
}
void push2(int u,int v,int id){
    redge[cnt].u=u;
    redge[cnt].v=v;
    redge[cnt].id=id;
    redge[cnt].next=rhead[u];
    rhead[u]=cnt;
    cnt++;
}
void tarjan(int fa){
    vis[fa]=true;
    for(int i=head[fa];i!=-1;i=edge[i].next){
        int v=edge[i].v;
        if(vis[v]){
            continue;
        }
        tarjan(v);
        Union(fa,v);
    }
    for(int i=rhead[fa];i!=-1;i=redge[i].next){
        int v=redge[i].v;
        if(vis[v]){
            ans[redge[i].id]=find(v);
        }
    }
}
int main(){
    int n,m,u,v;
    memset(head,-1,sizeof(head));
    memset(rhead,-1,sizeof(rhead));
    memset(fa,-1,sizeof(fa));
    memset(vis,0,sizeof(vis));
    scanf("%d%d",&n,&m);
    n-=1;
    while(n--){
        scanf("%d%d",&u,&v);
        push1(u,v);
        push1(v,u);
    }
    cnt=0;
    for(int i=1;i<=m;i++){
        scanf("%d%d",&u,&v);
        push2(u,v,i);
        push2(v,u,i);
    }
    tarjan(1);
    for(int i=1;i<=m;i++){
        printf("%d\n",ans[i]);
    }
    return 0;
}