class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        slow = fast = head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                return True
            else:
                continue
        return False