题目：

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

Follow up:
Can you solve it without using extra space?

这是一道关于链表比较简单的题，很顺利就解决了，不多说啦，上代码啦

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param head, a ListNode
    # @return a boolean
    def hasCycle(self, head):
        if head==None:
            check=False
        else:
            check=True
            node=head
            while(node.val!=0):
                node.val=0
                node=node.next
                if node==None:
                    check=False
                    break
        return check