Linked List Cycle Python Solution with Tests

Tested Python solution for LeetCode 141 with 15 pytest cases. Generate a practice environment with lcpy.

LeetCode 141, Easy. Topics: Hash Table, Linked List, Two Pointers. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 15 parametrized pytest cases, and a playground notebook:

lcpy gen -n 141   # by problem number
lcpy gen -s linked_list_cycle   # by problem name

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Problem

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

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Examples

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

Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Input: head = [1,2], pos = 0
Output: true

Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Input: head = [1], pos = -1
Output: false

Explanation: There is no cycle in the linked list.

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Constraints

• The number of the nodes in the list is in the range [0, 10^4].
• -10^5 <= Node.val <= 10^5
• pos is -1 or a valid index in the linked-list.

Follow up: Can you solve it using O(1) (i.e. constant) memory?

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

from leetcode_py import ListNode


class Solution:
    # Time: O(n)
    # Space: O(1)
    def has_cycle(self, head: ListNode[int] | None) -> bool:
        fast = head
        slow = head

        while fast and fast.next:
            assert slow is not None
            fast = fast.next.next
            slow = slow.next
            if fast is slow:
                return True

        return False

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Complexity

Time	Space
O(n)	O(1)

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Tags

Grind 75, Grind, Blind 75, NeetCode 150, NeetCode 250, NeetCode All.