版本分支（蓝桥）

package 第九届javab;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

public class 版本分支_树 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int Q = sc.nextInt();
		TreeNode trees[] = new TreeNode[N+1];
		for(int i = 1;i<=N;i++)
			trees[i] = new TreeNode(i);//初始化树
		int arr[][] = new int[N-1][2];
		HashSet<Integer> i0 = new HashSet<Integer>();
		HashSet<Integer> i1 = new HashSet<Integer>();
		for(int i = 0;i<arr.length;i++){
			arr[i][0] = sc.nextInt();
			arr[i][1] = sc.nextInt();
			i0.add(arr[i][0]);
			i1.add(arr[i][1]);
		}
		i0.removeAll(i1);
		int rot=0;
		for(int i:i0)
			rot = i;//寻找根节点
		System.out.println(rot);
		MyTree myTree = new MyTree(trees[rot]);
		for(int i = 0;i<arr.length;i++){
			myTree.insert(trees[arr[i][0]],trees[arr[i][1]]);//树的构建
		}
		myTree.print();
		for(int i = 0;i<Q;i++){
			int x = sc.nextInt();
			int y = sc.nextInt();
			f(trees[x], trees[y]);
		}
		
	}
	static void f(TreeNode parent,TreeNode child){
		HashSet<TreeNode> hashSet = new HashSet<TreeNode>();
		hashSet.add(child);
		while(child.parent!=null){
			hashSet.add(child.parent);
			child = child.parent;
		}
		if(hashSet.contains(parent)){
			System.out.println("YES");
		}else
			System.out.println("NO");
		
	}

}
class MyTree{
	TreeNode root;
	int size=0;
	public MyTree(TreeNode root){
		this.root = root;
		size++;
	}
	public void insert(TreeNode p,TreeNode child){
		if(p.children==null)
			p.children = new ArrayList<TreeNode>();
		p.children.add(child);
		child.parent = p;
	}
	public void print(){
		Queue<TreeNode>  queue  = new LinkedList<TreeNode>();
		queue.add(root);
		while(!queue.isEmpty()){
			TreeNode tmp = queue.poll();
			if(tmp.children!=null)
				for(TreeNode iMyTree:tmp.children)
					queue.add(iMyTree);
			System.out.println(tmp);
		}
	}
}


class TreeNode{
	int data;
	TreeNode parent;
	ArrayList<TreeNode> children;
	public TreeNode(int data){
		this.data = data;
	}
	@Override
	public String toString() {
		return "TreeNode [data=" + data + "]";
	}
	
}