聽了教練的考前須知 蒟蒻緊張的要死 只想做信心題

同時滿足：c或者d在x子樹裡 a或者b在y子樹裡 其中x=lca(a,b),y=lca(c,d)

#include<bits/stdc++.h>
#define N 100005
using namespace std;
template<class T>
inline void read(T &x)
{
    x=0;
    static char ch=getchar();
    while(!isdigit(ch)) ch=getchar();
    while(isdigit(ch))  x=x*10+ch-'0',ch=getchar();
}
struct Edge
{
    int to,next;
}edge[2*N];
int n,Q,first[N],tot;
inline void addedge(int x,int y)
{
    tot++;
    edge[tot].to=y; edge[tot].next=first[x]; first[x]=tot;
}
int depth[N],father[N],size[N],maxson[N];
void dfs1(int now,int fa)
{
    father[now]=fa;
    depth[now]=depth[fa]+1;
    size[now]=1;
    for(int u=first[now];u;u=edge[u].next)
    {
        int vis=edge[u].to;
        if(vis==fa) continue;
        dfs1(vis,now);
        size[now]+=size[vis];
        if(size[maxson[now]]<size[vis]) maxson[now]=vis;
    }
}
int top[N],dfn[N],sign;
void dfs2(int now,int topf)
{
    top[now]=topf;
    dfn[now]=++sign;
    if(maxson[now]) dfs2(maxson[now],topf);
    for(int u=first[now];u;u=edge[u].next)
    {
        int vis=edge[u].to;
        if(vis==father[now]||vis==maxson[now])  continue;
        dfs2(vis,vis);
    }
}
inline int lca(int x,int y)
{
    while(top[x]!=top[y])
    {
        if(depth[top[x]]<depth[top[y]]) swap(x,y);
        x=father[top[x]];
    }
    if(depth[x]<depth[y])   swap(x,y);
    return y;
}
int main()
{
    read(n),read(Q);
    for(int i=1,x,y;i<n;i++)
    {
        read(x),read(y);
        addedge(x,y); addedge(y,x);
    }
    dfs1(1,0);  dfs2(1,1);
    while(Q--)
    {
        int flag=0;
        int a,b,c,d,x,y;
        read(a),read(b),read(c),read(d);
        x=lca(a,b); y=lca(c,d);
        if((dfn[x]<=dfn[c]&&dfn[c]<=dfn[x]+size[x]-1)||(dfn[x]<=dfn[d]&&dfn[d]<=dfn[x]+size[x]-1))
            flag++; //如果c或者d在x子樹裡 
        if((dfn[y]<=dfn[a]&&dfn[a]<=dfn[y]+size[y]-1)||(dfn[y]<=dfn[b]&&dfn[b]<=dfn[y]+size[y]-1))
            flag++;//如果a或者b在y子樹裡 
        (flag==2)?puts("Y"):puts("N");
    }
}