Linked List Cycle#

Leetcode 141.Linked List Cycle Given a linked list, determine if it has a cycle in it. To represent a cycle in the given linked list, we use an integerposwhich represents the position (0-indexed)in the linked list where tail connects to. Ifposis-1, then there is no cycle in the linked list.

Example :

Input: head = [3,2,0,-4], pos = 1

Output: true

Explanation: There is a cycle in the linked list, where tail connects to the second node.

题意：给定一个链表，判断链表中是否有环。

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# Author:kilien
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# Leetcode 141.Linked List Cycle
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# 思路1：使用set记录链表元素，重复即有环
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# time:O(n) space:O(n)
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class Solution(object):
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    def hasCycle(self, head):
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        """
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        :type head: ListNode
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        :rtype: bool
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        """
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        key = set()
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        while head:
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            if head in key:
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                return True
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            else:
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                key.add(head)
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            head = head.next
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        return False
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# 思路2：使用快慢指针，当两者相遇则有环
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# time：O(n) space:O(1)
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# Definition for singly-linked list.
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# class ListNode(object):
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#     def __init__(self, x):
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#         self.val = x
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#         self.next = None
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class Solution(object):
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    def hasCycle(self, head):
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        """
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        :type head: ListNode
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        :rtype: bool
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        """
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        fast = slow = head
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        while fast and slow and fast.next:
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            fast = fast.next.next
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            slow = slow.next
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            if fast is slow:
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                return True
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        return False