Binary tree inorder iterator

#include <iostream>
#include <stack>
#include <vector>

struct TreeNode
{
	int data;
	TreeNode* left;
	TreeNode* right;

	TreeNode() {}

	TreeNode(int d, TreeNode* l, TreeNode* r)
	{
		data = d;
		left = l;
		right = r;
	}
};

class TreeNodeIterator
{
public:
	TreeNode* operator*() { return curNode; }
	TreeNodeIterator(TreeNode* root)
	{
		if (root) 
		{
			while (root)
			{
				stk.push(root);
				root = root->left;
			}
			curNode = stk.top();
		}
		else
		{
			curNode = NULL;
		}
	}

	bool hasNext() { return !stk.empty(); }

	TreeNode* getCur()
	{
		return curNode;
	}

	TreeNode* next() 
	{
		if (!hasNext()) return NULL;

		TreeNode* cur = stk.top();
		stk.pop();
		// if current node has right child
		cur = cur->right;
		if (cur)
		{
			while (cur)
			{
				stk.push(cur);
				cur = cur->left;
			}
		}

		if (!stk.empty())
		{
			curNode = stk.top();
			return stk.top();
		}
		else
			return NULL;
	}

private:
	std::stack<TreeNode*> stk;
	TreeNode* curNode;
};



int main()
{
	TreeNode* n6 = new TreeNode(6, NULL, NULL);
	TreeNode* n5 = new TreeNode(5, n6, NULL);
	TreeNode* n4 = new TreeNode(4, n5, NULL);
	TreeNode* n3 = new TreeNode(3, NULL, n4);
	TreeNode* n2 = new TreeNode(2, n3, NULL);
	TreeNode* n1 = new TreeNode(1, n2, NULL);

	TreeNodeIterator it(n1);
	while (it.hasNext())
	{
		std::cout << it.getCur()->data << " ";
		it.next();
	}
	return 0;
}