Binary Tree Zigzag Level Order Traversal

Description:

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

Code:

vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
       deque<TreeNode*>a;
       deque<TreeNode*>b;
       if (root)
            a.push_back(root);
        
       TreeNode*p = NULL;
       vector<vector<int>> result;
       
       while (!a.empty() || !b.empty())
       {
           vector<int>temp;
           if (!a.empty() )
           {
                while (!a.empty() )
                {
               p = a.front();
               a.pop_front();
               temp.push_back(p->val);
               if (p->left)
                    b.push_back(p->left);
                if (p->right)
                    b.push_back(p->right);
                }
                result.push_back(temp);
           }
           else
           {
            while (!b.empty())
           {
               p = b.front();
               b.pop_front();
               temp.push_back(p->val);
               if (p->left)
                    a.push_back(p->left);
               if (p->right)
                    a.push_back(p->right);
           }
           result.push_back(temp);
           }
       }
       for (int i = 1; i < result.size(); i+=2)
       {
           reverse(result[i].begin(),result[i].end());
       }
       return result; 
    }