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
# 采用集合判断是否重复访问节点
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        this = head
        visit = {head}
        while this:
            this = this.next
            if this in visit:
                return True
            visit.add(this)
        return False