text: # 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):
          """
          :type head: ListNode
          :rtype: bool
          """
          slow = head
          fast = head
          while slow and fast and fast.next:
              slow = slow.next
              fast = fast.next.next
              if slow is fast:
                  return True
          return False
  