Complete the code for a function that inserts a node with info
     value x before, and a node with info value zafter,each node with
     info value y in a linked list with first node pointer p. Assume the info
     type is int and that x,y and z contain distinct.  
     The function returns a pointer to the first node of the
     modified list.

Node*  InsertBeforeAndAfter(Node* p, int x, int y, int z)

Utility function:

Node *  MakeNode(intnewVal, Node* successor)

{   Node * tmp = malloc(sizeof(Node));

    assert(tmp != NULL);

    tmp->info = newVal; tmp->next = successor;

    return tmp;

}