Binary Tree Level Order Traversal leetcode java

題目：

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]
]


題解：
這道題就是用傳統的BFS來做。代碼如下：

 
           1     
          public ArrayList<ArrayList<Integer>> levelOrder(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          
          int curLevCnt = 1;   
          
 
           9          
          int nextLevCnt = 0;   
          
 
          10         ArrayList<Integer> levelres =  
          new ArrayList<Integer>();   
          
 
          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<Integer>();   
          
 
          31             }   
          
 
          32         }   
          
 
          33          
          return res;   
          
 
          34     }