剑指Offer----从上往下打印二叉树

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.LinkedList;import java.util.Queue;/**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>();        if (root==null){            return arrayList;        }        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.add(root);        while (!queue.isEmpty()){            TreeNode node= queue.poll();            arrayList.add(node.val);            if (node.left!=null){                queue.add(node.left);            }            if (node.right!=null){                queue.add(node.right);            }        }        return arrayList;    }}