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文章作者：Tyan

1. 问题描述

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:

3   / \  9  20    /  \   15   7

return its level order traversal as:

[  [3],  [9,20],  [15,7]]

2. 求解

这个题就是一个树的层次遍历问题，需要用到新的数据结构队列，把每一层的结点的子结点放入到队列中，依次遍历。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List
   
    
     > levelOrder(TreeNode root) {        List
     
      
       > list = new ArrayList
       
        
         >(); if(root == null) { return list; } Queue
         
           queue = new LinkedList
          
           (); queue.add(root); Queue
           
             result = new LinkedList
            
             (); List
             
               level = new ArrayList
              
               (); while(!queue.isEmpty()) { TreeNode temp = queue.poll(); level.add(temp.val); if(temp.left != null) { result.add(temp.left); } if(temp.right != null) { result.add(temp.right); } if(queue.isEmpty()) { queue = result; result = new LinkedList
               
                (); list.add(level); level = new ArrayList
                
                 (); } } return list; }}
                
               
              
             
            
           
          
         
        
       
      
     
    
   

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