/** * 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 */ package javabasic.nowcoder; import java.util.ArrayList; import java.util.Queue; import javax.management.Query; class TreeNode27 { int val = 0; TreeNode left = null; TreeNode right = null; public TreeNode27(int val) { this.val = val; } } /* * 借组一个队列辅助层次遍历二叉树 */ public class Main27 { public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) { ArrayList<Integer> arr = new ArrayList<Integer>(); if(root==null) { return arr; } Queue<TreeNode> queue = (Queue<TreeNode>) new LinkedList(); queue.offer(root); while(!queue.isEmpty()) { TreeNode temp = queue.poll(); if(temp.left!=null) { queue.offer(temp.left); } if(temp.right!=null) { queue.offer(temp.right); } arr.add(temp.val); } return arr; } }