144. Binary Tree Preorder Traver

Given a binary tree, return the preorder traversal of its nodes' values.
For example:Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> path;
        TreeNode* p = NULL;
        if(root == NULL)
          return path;
        stack<TreeNode*> s;
        s.push(root);
        while(!s.empty())
        {
            p = s.top();
            path.push_back(p->val);
            s.pop();
            if(p->right)
              s.push(p->right);
            if(p->left)
              s.push(p->left);
        }
        return path;
    }
};