141. Linked List Cycle

 Description:

Given a linked list, determine if it has a cycle in it.

给定一个链表，确定出是否含有环

Solutions:

思路一：快指针和慢指针方法

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if head is None:
            return False
        fast=head
        slow=head
        
        while fast.next is not None and fast.next.next is not None:
            slow=slow.next
            fast=fast.next.next
            
            if slow==fast:
                return True
        return False