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boolean checkBST(Node root) {
        return checkHelper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    boolean checkHelper(Node node, int min_left, int max_right) {
        if (node == null) {
            return true;
        }
        
        if (node.data <= min_left || node.data >= max_right) {
            return false;
        }
        
        return checkHelper(node.left, min_left, node.data) && checkHelper(node.right, node.data, max_right);
    }