linked_list_cycle.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        if not head or not head.next:
            return False
        slow_ll = head
        fast_ll = slow_ll.next
        
        while fast_ll and fast_ll.next:
            if slow_ll == fast_ll:
                return True
            slow_ll = slow_ll.next
            fast_ll = fast_ll.next.next
        
        return False