new_contents: """
  Given a linked list, determine if it has a cycle in it.
  
  Follow up:
  Can you solve it without using extra space?
  
  """
  
  # Definition for singly-linked list.
  # class ListNode(object):
  #     def __init__(self, x):
  #         self.val = x
  #         self.next = None
  
  
  class Solution(object):
      def hasCycle(self, head):
          if not head:
              return False
          slow = head
          fast = head
          while fast and fast.next:
              fast = fast.next.next
              slow = slow.next
              if fast == slow:
                  return True
          return False
subject: add linked list cycle