打卡题目：leetcode第141题，环形链表
    相似题目：142
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    【代码】

    1. class Solution:
    2.     def detectCycle(self, head: ListNode) -> ListNode:
    3.         fast, slow = head, head
    4.         while fast != None:
    5.             fast = fast.next
    6.             if fast == None:
    7.                 break
    8.             fast = fast.next
    9.             slow = slow.next
    10.             if slow == fast:
    11.                 break
    12.            if fast == None:
    13.             return None
    14.         p1, p2 = slow, head
    15.         while p1 != p2:
    16.             p1 = p1.next
    17.             p2 = p2.next
    18.         return p1