从上到下打印二叉树 层序遍历

从上到下打印二叉树  层序遍历

package com.java.offer;

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

class TreeNode{
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode(int x){
		this.val=x;
	}
}

public class Solution06 {
	//从上到下打印二叉树  层序遍历
	public ArrayList<Integer> printFromToBottom(TreeNode root){
		if(root==null){
			return null;
		}
		ArrayList<Integer> list=new ArrayList<Integer>();
		//先打印左子树 再打印右子树
		//借助队列
		Queue<TreeNode>  queue=new LinkedList<TreeNode>();
		queue.offer(root);
		while(!queue.isEmpty()){
			TreeNode node=queue.poll();
			if(node.left!=null){
				queue.offer(node.left);
			}
			if(node.right!=null){
				queue.offer(node.right);
			}
			list.add(node.val);
		}
		return list;
	}
}