[leetcode] Binary Tree Preorder Traversal

public List<Integer> preorderTraversal(TreeNode root) {
	List<Integer> result = new ArrayList<Integer>();
	Stack<TreeNode> st = new Stack<TreeNode>();
	TreeNode current = root;
	
	while(current != null || !st.isEmpty())
	{
		if(current != null)
		{
			result.add(current.val);//Add parent
			st.push(current.right);
			current = current.left;
		}
		else
		{
			current = st.pop();
		}
	}
	return result;
}