文章

LeetCode Linked List Cycle

Problem

Given a linked list, determine if it has a cycle in it.

Follow up: Can you solve it without using extra space?

判断一个链表中是否存在 环,不开辟额外的空间

Python 实现

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
# Given a linked list, determine if it has a cycle in it.
#
# Follow up:
# Can you solve it without using extra space?

# 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 is None:
            return False
        slow, faster = head, head.next
        while faster is not None:
            slow = slow.next
            if faster.next is not None:
                faster = faster.next.next
            else:
                return False
            if slow is faster:
                return True
        return False

class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None


head = ListNode(1)
two = ListNode(2)
two.next = head
head.next = two

print(Solution().hasCycle(head))

分析

题目要求不适用额外的内存空间。所以,其实我想到两个办法,一个是判断该节点是否被访问过。可以把所有访问过的节点,都变成head节点,这样,当你.next又访问到head节点时,那说明有环。(因为访问到了之前访问过的节点。)不过,我并没有采用这个方法,因为它改变了链表数据,不具备实际作用。

既然是环,就采用了追击的办法,两个指针,一个快(2步),一慢(1步),如果是环,则总会追上。效率O(n)。

本文由作者按照 CC BY 4.0 进行授权