题解 | 判断链表中是否有环

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

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        if head is None:
            return False
        object_set = set()
        cur_node = head
        while (cur_node.next):
            if cur_node in object_set:
                return True
            object_set.add(cur_node)
            cur_node = cur_node.next
        return False