Binary Tree Level Order Traversal leetcode java

タイトル:
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]
]


  ：
         BFS  。    ：

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