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 not head:
            return False
        elif not head.next:
            return False
        else:
            q_flag, s_flag = head.next, head
            while q_flag and s_flag:
                if q_flag == s_flag:
                    return True
                #   快指针
                q_flag = q_flag.next if not q_flag.next else q_flag.next.next
                #   慢指针
                s_flag = s_flag.next
            return False