从上往下打印二叉树

Description：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;/**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> list = new ArrayList<>();        LinkedList<TreeNode> ll = new LinkedList<>();        if (root == null) return list;        ll.offer(root);        while (ll.size() > 0) {            TreeNode node = ll.pop();            list.add(node.val);            if (node.left != null) ll.offer(node.left);            if (node.right != null) ll.offer(node.right);        }        return list;    }}