cycle-detection.py

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

# https://leetcode.com/problems/linked-list-cycle/description/

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        if head is None:
            return False

        visited = {}
        while head.next:
            if head in visited:
                return True
            visited[head] = True
            head = head.next
        return False