LeetCode 141. Linked List Cycle

linked_list_cycle_detection.py

from typing import Optional


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


def hasCycle(head: Optional[ListNode]) -> bool:
    """
    LeetCode 141. Link List Cycle
    """
    tortoise = hare = head
    while hare and hare.next:
        tortoise = tortoise.next
        hare = hare.next.next
        if tortoise is hare:
            return True
    return False

This is partial implementation of Floyd's "Tortoise and Hare" algorithm, which in full, calculates the cycle length if a cycle is present.