private void deleteCase1(Node doubleBlackNode, AtomicReference<Node> rootReference) {
        if(doubleBlackNode.parent == null) {
            rootReference.set(doubleBlackNode);
            return;
        }
        deleteCase2(doubleBlackNode, rootReference);
    }
If double black node becomes root then we are done. Turning it into
single black node just reduces one black in every path.
src/com/interview/tree/RedBlackTree.java