Circular singly linked list
Nice to solve before
Instructions
Given a linked list, return true if the list is circular, false if it is not.
Examples
Example 1
val l = new SinglyLinkedList ()
val a = new Node ('a')
val b = new Node ('b')
val c = new Node ('c')
l.head = a
a.next = b
b.next = c
c.next = b
circular(l) // true
Example 2
val l = new List ()
val a = new Node ('a')
circular(l) // false
Hints
Hint 1
We should use more then one variable to store values that are retrieved during iteration (two pointer solution)
Hint 2
Name of these variables should be `slow` and `fast`
Hint 3
Assign next node to `slow` variable in every iteration
Hint 4
Assign next node of next node to `fast` variable in every iteration