public List<Integer> inorderTraversal(TreeNode root) {
		List<Integer> ans = new ArrayList<>();
		if (root == null)
			return ans;
		Stack<TreeNode> stack = new Stack<>();
		while (root != null) {
			stack.push(root);
			root = root.left;
		}
		while (!stack.isEmpty()) {
			TreeNode node = stack.pop();
			ans.add(node.val);
			TreeNode right = node.right;
			while (right != null) {
				stack.push(right);
				right = right.left; // get left of the right node
			}
		}
		return ans;
	}