linked list cycle

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

用tortoise and hare算法，一个快指针一个慢指针。

代码块

leetcode上的最优解法：

# 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 """

        try:
            slow=head
            fast=head.next
            while slow is not fast:
                slow=slow.next
                fast=fast.next.next
            return True
        except:
            return False

自己的：

class Solution(object):
    def hasCycle(self, head):
        """ :type head: ListNode :rtype: bool """
        if head == None or head.next == None:
            return False

        slow = head
        fast = head.next

        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
            if fast == slow:
                return True
        return False

StackEdit