[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 ListNodeclass 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)。