#!/usr/bin/env python
#-*- coding: UTF-8 -*-
###########################################################################
#
# Copyright (c) 2018 www.codingchen.com, Inc. All Rights Reserved
#
##########################################################################
'''
  @brief leetcode algorithm
  @author chenhui
  @date 2018-11-29 11:05:08
'''
import json

# 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 = fast = head
        while slow and fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True
        return False