private Optional<Node> findSiblingNode(Node root) {
        Node parent = root.parent;
        if(isLeftChild(root)) {
            return Optional.ofNullable(parent.right.isNullLeaf ? null : parent.right);
        } else {
            return Optional.ofNullable(parent.left.isNullLeaf ? null : parent.left);
        }
    }
Main validate method of red black tree.
src/com/interview/tree/RedBlackTree.java