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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,null,null,15,7],

return its bottom-up level order traversal as: [[15,7],[9,20],[3]]

队列

从第一层开始，每次读一个sublist，读完之后将其弹出，再将新一层的节点读入，像队列一样，代码如下：

public class Solution {
       public List
   
    
     > levelOrderBottom(TreeNode root) {        List
     
      
       > list = new ArrayList
       
        
         > (); if(root == null) return list; Queue
         
           queue = new LinkedList
          
           (); queue.add(root); while(queue.size() > 0) { List
           
             sublist = new ArrayList
            
             (); int tempSize = queue.size(); for(int i = 0; i < tempSize; i++) { TreeNode node = queue.poll(); sublist.add(node.val); if(node.left != null) queue.add(node.left); if(node.right != null) queue.add(node.right); } list.add(0, sublist); } return list; }}
            
           
          
         
        
       
      
     
    
   

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