input: Write a python function to detect whether a linked list contains a cycle.
  assert detect_cycle(node_with_cycle) == True
  assert detect_cycle(node_without_cycle) == False
output: def detect_cycle(head):
      slow = head
      fast = head
      
      while fast and fast.next:
          slow = slow.next
          fast = fast.next.next
          
          if slow == fast:
              return True
              
      return False