leetcode之Linked List Cycle

# 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
"""
if not head:
return False
if not head.next:
return False
twostep = head.next.next
for i in xrange(2147483647):
if head.next == twostep:
return True
else:
head = head.next
if not twostep:
return False
elif not twostep.next:
return False
else:
twostep = twostep.next.next