[LintCode 102] 带环链表(Python)

题目描述

给定一个链表，判断它是否有环。

样例

给出 -21->10->4->5, tail connects to node index 1，返回 true

思路

快慢针。快针每次都两步，慢针每次走一步。如果无环，快针肯定会率先到达队尾，即遇到None。如果有环，快针永远无法遇到None，并且会与慢针相遇。

代码

"""
Definition of ListNode
class ListNode(object):

def __init__(self, val, next=None):
self.val = val
self.next = next
"""

class Solution:
"""
@param head: The first node of the linked list.
@return: True if it has a cycle, or false
"""
def hasCycle(self, head):
# write your code here
if head is None:
return False
f = head
s = head
while f.next is not None and f.next.next is not None:
f = f.next.next
s = s.next
if s == f:
return True
return False

复杂度分析

时间复杂度O(n)，空间复杂度O(1)。