[leetcode] Binary Tree Level Order Traversal

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},

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]
给定一棵二叉树，对其进行层次遍历。使用一个队列，加入一个NULL标记层次，每次从队首取出一个节点，然后存入结果，并把他的孩子加入队尾，直到队列为空。
代码如下：

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<vector<int> > levelOrder(TreeNode *root) {
13         // Note: The Solution object is instantiated only once and is reused by each test case.
14         queue<TreeNode*> q;
15         q.push(NULL);
16         vector<vector<int>> result;
17         vector<int> temp;
18         if(root == NULL)return result;
19         else
20         {
21             q.push(root);
22             while(q.size()!=1)
23             {
24              while(q.front()!=NULL)
25              {
26                  temp.push_back(q.front()->val);
27                  if(q.front()->left!=NULL) q.push(q.front()->left);
28                  if(q.front()->right!=NULL) q.push(q.front()->right);
29                  q.pop();
30              }
31              if(q.front()==NULL)
32              {
33                q.pop();
34                q.push(NULL);
35              }
36              if(temp.size()!=0) result.push_back(temp);
37              temp.clear();
38             }
39         }
40      return result;   
41     }
42 };