Binary Tree Right Side View

Description:

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:
Given the following binary tree,

   1            <---
 /   
2     3         <---
      
  5     4       <---

You should return [1, 3, 4].

Code:

   vector<int> rightSideView(TreeNode *root) {
        deque<TreeNode*>a;
        deque<TreeNode*>b;
        vector<int>result;
        
        if (root==NULL)
            return result;
            
        a.push_back(root);
        while ( !a.empty() || !b.empty() )
        {
            if ( !a.empty() )
            {
                //去队首元素
                result.push_back(a.front()->val);
                //将a中所有元素的孩子存放到b中,并清空a
                while (!a.empty())
                {
                    TreeNode* p = a.front();
                    if (p->right)
                        b.push_back(p->right);
                    if (p->left)
                        b.push_back(p->left);
                    a.pop_front();
                }
            }
            else
            {
                result.push_back(b.front()->val);
                //将a中所有元素的孩子存放到b中,并清空a
                while (!b.empty())
                {
                    TreeNode* p = b.front();
                    if (p->right)
                        a.push_back(p->right);
                    if (p->left)
                        a.push_back(p->left);
                    b.pop_front();
                }
            }
        }
         return result;
    }