题目

题解

思路：floyd判圈法；

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
                fast, slow = head, head
                while True:
                    if not (fast and fast.next):
                        return
                    fast = fast.next.next
                    slow = slow.next
                    if fast == slow:
                        break
                fast = head
                while fast != slow:
                    fast = fast.next
                    slow = slow.next
                return fast