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

For example:

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

3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

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

 

/** * 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
     
      
       > levelOrderBottom(TreeNode root) {        List
       
        
         > res = new ArrayList
         
          
           >(); Stack
           
            
             > stack = new Stack
             
              
               >(); Queue
               
                 q = new LinkedList
                
                 (); if(root == null) return res; q.add(root); while(!q.isEmpty()){ List
                 
                   tmp = new ArrayList
                  
                   (); int size = q.size(); for(int i = 0; i< size; i++){ TreeNode node = q.poll(); tmp.add(node.val); if(node.left != null) q.add(node.left); if(node.right != null) q.add(node.right); } stack.push(tmp); } while(!stack.isEmpty()){ res.add(stack.pop()); } return res; }}