Binary Tree Level Order Traversal II @LeetCode

package Level3;  
  
import java.util.ArrayList;  
import java.util.LinkedList;  
import java.util.Queue;  
  
import Utility.TreeNode;  
  
  
/** 
 * Binary Tree Level Order Traversal II  
 *  
 *  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  > read more on how binary tree is serialized on OJ. 


OJ's Binary Tree Serialization: 
The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below. 

Here's an example: 
   1 
  / \ 
 2   3 
    / 
   4 
    \ 
     5 
The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}". 
 * 
 */  
public class S107 {  
  
    public static void main(String[] args) {  
  
    }  
  
    public ArrayList> levelOrderBottom(TreeNode root) {  
        ArrayList> ret = new ArrayList>();  
          
        if(root == null){  
            return ret;  
        }  
        Queue





 queue = new LinkedList();  
        queue.add(root);  
          
        ArrayList> alal = new ArrayList>();  
        ArrayList al = new ArrayList();  
          
        // 记录两个当前level和下一个level的node数目  
        int currentLevel = 1;  
        int nextLevel = 0;  
          
        while( !queue.isEmpty() ){  
            TreeNode cur = queue.remove();  
            currentLevel--;  
            al.add(cur.val);  
              
            if(cur.left != null){  
                queue.add(cur.left);  
                nextLevel++;  
            }  
            if(cur.right != null){  
                queue.add(cur.right);  
                nextLevel++;  
            }  
              
            if(currentLevel == 0){  
                alal.add(al);  
                al = new ArrayList();  
                currentLevel = nextLevel;  
                nextLevel = 0;  
            }  
        }  
          
        // 翻转ArrayList  
        for(int i=alal.size()-1; i>=0; i--){  
            ret.add(alal.get(i));  
        }  
          
        return ret;  
    }  
  
}