标签：没有 inf 列表 pytho node lin 还需 bool 最大值
问题描述：
Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer poswhich represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Example 1:
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.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
思路1：
设置两个指针，一个每次走一步，一个每次走两步，那么如果有环，两个指针一定会相遇。
但这个代码写出来仅仅beats 28.6%，效率不高
代码1:
# 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 head == None or head.next == None: return False head2 = head while head2.next != None and head.next != None:#当两个指针的next都不为None if (head2.next.next != None):#如果走两步的指针head2的next.next不为None head2 = head2.next.next else: return False head = head.next if(head == head2): return True return False
将上述思路优化后，代码如下：
if not head or not head.next: return False head2 = head while not head2 and not head2.next:#只需要关注走的比较快的指针是否为空即可，若较快指针不为空，则较慢指针肯定不为空 head2 = head2.next.next head = head.next if(head == head2): return True return False
思路2：
循环遍历整个列表，将遍历过的节点值设置为inf(最大值），如果后续遍历的节点值有等于inf的，则证明有环，否则就是没有环。
该算法写出来beats65.7%的人，还需要优化
代码2：
# 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 head == None or head.next == None: return False head.val = float(‘inf‘) while head.next != None:#当指针的next都不为None if head.val == head.next.val: return True else: head = head.next head.val = float(‘inf‘) return False
Python3解leetcode Linked List Cycle
标签：没有 inf 列表 pytho node lin 还需 bool 最大值
原文地址：https://www.cnblogs.com/xiaohua92/p/11080739.html
