m142 v2 O(n) runtime.py

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

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head :
            return None
        
        past = set()

        while head :
            if head in past :
                return head
            
            past.add(head)
            head = head.next
        return None