java面试题（剑指offer）：按层打印二叉树

从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

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

class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}

public class ddd {
    
    
	public ArrayList > Print(TreeNode pRoot) {
        ArrayList> alist=new ArrayList>();
        Queue qu=new LinkedList();
        Queue qu2=new LinkedList();
		if(pRoot==null){
			return alist;
		}
		qu.add(pRoot);
		while(!qu2.isEmpty() || !qu.isEmpty()){
            ArrayList list=new ArrayList();
			while(!qu.isEmpty() &&qu.peek()!=null){
				qu2.add(qu.peek());
				list.add(qu.peek().val);
				qu.remove();
			}
			alist.add(list);
			while(!qu2.isEmpty() && qu2.peek()!=null){
				if(qu2.peek().left!=null){
					qu.add(qu2.peek().left);
				}
				if(qu2.peek().right!=null){
					qu.add(qu2.peek().right);
				}
				qu2.remove();
			}
		}
		return alist;
    }
}