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:
        post = head
        if post == None:
            return False
        pre = head.next
        if pre == None :
            return False
        while pre:
            if pre.next == post:
                return True
            pre = pre.next
            if pre == None:
                return False
            pre = pre.next
            post = post.next
        return False