描述
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
分析
经典题，使用两指针，一个每次走一步，另一个走两步，如果两指针相遇，则说明有环。
代码
Python
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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
"""
if not head or not head.next:
return False
p1 = head.next
p2 = head.next.next
while p2:
if not p2 or not p2.next:
return False
if p1 == p2:
return True
p2 = p2.next.next
p1 = p1.next
return False
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