【leetcode】141. 环形链表（python）

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

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        nums = set()
        while head:
            if head in nums:
                return True
            else:
                nums.add(head)   # note: not add head.val
                head = head.next
        return False