本文共 967 字，大约阅读时间需要 3 分钟。

思路：逐层把TreeNode添加到list中然后逐层读取其val。

/** * 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
     
      
       > answerList = new ArrayList
       
        
         >(); List
         
          
           > list = new ArrayList
           
            
             >(); if (root == null) { return list; } List
             
               initList = new ArrayList
              
               (); initList.add(root); answerList.add(initList); for (int i = 0; i < answerList.size(); i++) { List
               
                 l = new ArrayList
                
                 (); List
                 
                   newList = new ArrayList
                  
                   (); for (int j = 0; j < answerList.get(i).size(); j++) { TreeNode tree = answerList.get(i).get(j); l.add(answerList.get(i).get(j).val); if (tree.left != null) { newList.add(tree.left); } if (tree.right != null) { newList.add(tree.right); } } list.add(l); if (newList.size() != 0) { answerList.add(newList); } } return list; }}
                  
                 
                
               
              
             
            
           
          
         
        
       
      
     
    
   

耗时：328ms，中游