题目
这个的想法就是用快慢指针的办法，一个walker,一个runner， walker一次走一步，runner一次走两步，然后如果它们能够相见的话说明有环。
# 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
"""
walker = head
runner = head
while (runner != None and runner.next != None):
walker = walker.next
runner = runner.next.next
if(walker==runner):
return True
return False
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