剑指offer--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。

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思路：二叉树的层次遍历

import java.util.ArrayList;/**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<>();        ArrayList<TreeNode> queue = new ArrayList<>();        if ( root == null){            return arrayList;        }        queue.add(root);        for ( int i = 0 ; i < queue.size() ; i++){            TreeNode node = queue.get(i);            if ( node.left != null){                queue.add(node.left);            }            if ( node.right != null){                queue.add(node.right);            }            arrayList.add(node.val);        }        return arrayList;    }}