0141-linked-list-cycle.py

# time complexity: O(n)
# space complexity: O(n)
from typing import Optional

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

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        if not head:
            return False

        slow = head
        fast = head.next

        while slow != fast:
            if fast is None or fast.next is None:
                return False
            fast = fast.next.next
            slow = slow.next
        return True