从上往下打印二叉树

题目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。题解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> result = new ArrayList<>(); if (root == null) { return result; } Queue<TreeNode> queue = new LinkedList<>(); queue.offer(root); while(!queue.isEmpty()) { TreeNode current = queue.poll(); TreeNode left = current.left; TreeNode right = current.right; if (left != null) { queue.offer(left); } if (right != null) { queue.offer(right); } result.add(current.val); } return result; }}