Binary Tree Level Order Traversal

Description:

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]
]

Code:

 1 vector<vector<int>> levelOrder(TreeNode* root) {
 2        deque<TreeNode*>a;
 3        deque<TreeNode*>b;
 4        if (root)
 5             a.push_back(root);
 6         
 7        TreeNode*p = NULL;
 8        vector<vector<int>> result;
 9        
10        while (!a.empty())
11        {
12            vector<int>temp;
13            if (!a.empty() )
14            {
15                 while (!a.empty() )
16                 {
17                p = a.front();
18                a.pop_front();
19                temp.push_back(p->val);
20                if (p->left)
21                     b.push_back(p->left);
22                 if (p->right)
23                     b.push_back(p->right);
24                 }
25                 result.push_back(temp);
26            }
27            
28            while (!b.empty())
29            {
30                a.push_back(b.front());
31                b.pop_front();
32            }
33        }
34        return result;
35     }