从上往下打印二叉树

题目描述

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

import java.util.ArrayList;
import java.util.Queue;
import java.util.LinkedList;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> arraylist = new ArrayList<Integer>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        if(root==null)
            return arraylist;
        //将根节点pRoot入队列
        queue.offer(root);
        while(!queue.isEmpty()){
            //只要队列不为空就执行下面的操作
            //首先将当前节点出队列
            TreeNode newtree = queue.poll();
            //判断是否存在左右节点，并各自入队列
            if(newtree.left!=null){
                queue.offer(newtree.left);
            }
            if(newtree.right!=null){
                queue.offer(newtree.right);
            }

            //将当前节点的值val添加进arraylist
            arraylist.add(newtree.val);
        }
        return arraylist;
    }
}