题解 | 判断链表中是否有环

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

#
# 
# @param head ListNode类 
# @return bool布尔型
#
#如果有环则返回true，否则返回false。
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        if head is None:
            return False
        fast=head
        slow=head
        while fast!=None and fast.next!=None:
            fast=fast.next.next
            slow=slow.next
            if fast==slow:
                return True
        return False            

        # if head is None:
        #     return False
        # Phead=head
        # fast=head
        # slow=head
        # while fast is not None and fast.next is not None:
        #     fast=fast.next.next
        #     slow=slow.next
        #     if fast==slow:
        #         return True
        # else:
        #     return False