python3题解 -快慢指针 | #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
        fast = head
        slow = head
        while fast != None and fast.next != None:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True
        return False