private void deleteCase2(Node doubleBlackNode, AtomicReference<Node> rootReference) {
        Node siblingNode = findSiblingNode(doubleBlackNode).get();
        if(siblingNode.color == Color.RED) {
            if(isLeftChild(siblingNode)) {
                rightRotate(siblingNode, true);
            } else {
                leftRotate(siblingNode, true);
            }
            if(siblingNode.parent == null) {
                rootReference.set(siblingNode);
            }
        }
        deleteCase3(doubleBlackNode, rootReference);
    }
If sibling is red and parent and sibling's children are black then rotate it
so that sibling becomes black. Double black node is still double black so we need
further processing.
src/com/interview/tree/RedBlackTree.java