Binary Tree Preorder Traversal

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> vecNode;
		stack<TreeNode* > sNode;
		TreeNode*  temp = root;

		while (0 != temp || !sNode.empty())
		{
			if (0 != temp)
			{
				vecNode.push_back(temp->val);
				if (0 != temp->right)
					sNode.push(temp->right);
				temp = temp->left;
			}
			else
			{
				temp = sNode.top();
				sNode.pop();
			}
		}
		return vecNode;
	}
};