问题描写叙述：

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

For example:

Given binary tree {3,9,20,#,#,15,7},

3   / \  9  20    /  \   15   7

return its level order traversal as:

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

confused what "{1,#,2,3}" means?

代码：

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {public List
     
      
       > levelOrder(TreeNode root) {  //java		if(root == null)			return new ArrayList<>();					Stack
       
         stack = new Stack
        
         ();		stack.add(root);		List
         
           levelList = new ArrayList
          
           (); List
           
            
             > result = new ArrayList
             
              
               >(); List
               
                
                 > invert_result = new ArrayList
                 
                  
                   >(); List
                   
                     tmp = new ArrayList
                    
                     (); while(!stack.isEmpty()|| !levelList.isEmpty()){ if(stack.isEmpty()){ for(int i=levelList.size()-1; i>=0; i--) stack.push(levelList.get(i)); levelList.clear(); invert_result.add(tmp); tmp = new ArrayList
                     
                      (); } while(!stack.isEmpty()){ TreeNode node = stack.pop(); tmp.add(node.val); if(node.left !=null) levelList.add(node.left); if(node.right !=null) levelList.add(node.right); } } invert_result.add(tmp); return invert_result; }}