洛谷 [P3398] 仓鼠找sugar

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstdlib>
using namespace std;
const int MAXN=200005;
int init(){
    int rv=0,fh=1;
    char c=getchar();
    while(c<'0'||c>'9'){
        if(c=='-') fh=-1;
        c=getchar();
    }
    while(c>='0'&&c<='9'){
        rv=(rv<<1)+(rv<<3)+c-'0';
        c=getchar();
    }
    return rv*fh;
}
int head[MAXN],dep[MAXN],fa[MAXN],siz[MAXN],son[MAXN],n,m,nume,top[MAXN],id[MAXN],ind;
struct edge{
    int to,nxt;
}e[MAXN<<1];
void adde(int from,int to){
    e[++nume].to=to;
    e[nume].nxt=head[from];
    head[from]=nume;
}
void dfs1(int u,int rt){
    fa[u]=rt;
    dep[u]=dep[rt]+1;
    siz[u]=1;
    int ma=0;
    for(int i=head[u];i;i=e[i].nxt){
        int v=e[i].to;
        if(v==rt) continue;
        dfs1(v,u);
        siz[u]+=siz[v];
        if(ma>siz[v]){
            ma=siz[v];
            son[u]=v;
        }
    }
}
void dfs2(int u,int topf){
    top[u]=topf;
    id[u]=++ind;
    if(!son[u]) return;
    dfs2(son[u],topf);
    for(int i=head[u];i;i=e[i].nxt){
        int v=e[i].to;
        if(v==fa[u]||v==son[u]) continue;
        dfs2(v,v);
    }
}
int LCA(int a,int b){
    while(top[a]!=top[b]){
        if(dep[top[a]]>dep[top[b]]) a=fa[top[a]];
        else b=fa[top[b]];
    }
    return dep[a]>dep[b]?b:a;
}
int main(){
    n=init();m=init();
    for(int i=1;i<n;i++){
        int u=init(),v=init();
        adde(u,v);adde(v,u);
    }
    dep[1]=1;
    dfs1(1,0);
    dfs2(1,1);
    for(int i=1;i<=m;i++){
        int a=init(),b=init(),c=init(),d=init();
        int r1=LCA(a,b),r2=LCA(c,d);
        if(dep[r1]<dep[r2]){
            swap(r1,r2);
            swap(a,c);
            swap(b,d);
        }
        if(LCA(r1,c)==r1||LCA(r1,d)==r1) cout<<"Y"<<endl;
        else cout<<"N"<<endl;
    }
    return 0;
}