Binary Tree Level Order Traversal II leetcode java

 1     public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
 2         ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();  
 3         if(root == null)  
 4             return res;
 5         
 6         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();  
 7         queue.add(root);
 8         
 9         int curLevCnt = 1;  
10         int nextLevCnt = 0;  
11         
12         ArrayList<Integer> levelres = new ArrayList<Integer>();  
13        
14         while(!queue.isEmpty()){  
15             TreeNode cur = queue.poll();  
16             curLevCnt--;  
17             levelres.add(cur.val);  
18             
19             if(cur.left != null){  
20                 queue.add(cur.left);  
21                 nextLevCnt++;  
22             }  
23             if(cur.right != null){  
24                 queue.add(cur.right);  
25                 nextLevCnt++;  
26             }  
27               
28             if(curLevCnt == 0){  
29                 curLevCnt = nextLevCnt;  
30                 nextLevCnt = 0;  
31                 res.add(0,levelres);  //insert one by one from the beginning
32                 levelres = new ArrayList<Integer>();  
33             }  
34         }  
35         return res;  
36     }