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 public int kthSmallest(TreeNode root, int k) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode current = root;

        while (!stack.isEmpty() || current != null) {
            // Traverse the left subtree
            while (current != null) {
                stack.push(current);
                current = current.left;
            }

            // Process the current node
            current = stack.pop();
            k--;
            if (k == 0) {
                return current.val;
            }

            // Traverse the right subtree
            current = current.right;
        }

        return -1; // If k is invalid
    }