题目描述：
Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Follow up:
Can you solve it using O(1) (i.e. constant) memory?
第一种实现方法：
1 def hasCycle(self, head): 2 try: 3 slow = head 4 fast = head.next 5 while slow is not fast: 6 slow = slow.next 7 fast = fast.next.next 8 return True 9 except: 10 return False
第二种实现方法：
1 class Solution(object): 2 def hasCycle(self, head): 3 marker1 = head 4 marker2 = head 5 while marker2!=None and marker2.next!=None: 6 marker1 = marker1.next 7 marker2 = marker2.next.next 8 if marker2==marker1: 9 return True 10 return False
分析：这道题可以看做是两个人在一个赛道上赛跑，其中一个人跑的快，另外一个人跑的慢。如果这个赛道有环，那么终究有一天，两个人都会进入环中。而在一个环中，只要两个人的速度不同，那么它们终将相遇。