题目链接：https://leetcode.cn/problems/linked-list-cycle-ii/
难度：中等

描述：
给定一个链表的头节点 head ，返回链表开始入环的第一个节点。 如果链表无环，则返回 _null_。

题解

  1. # Definition for singly-linked list.
  2. # class ListNode:
  3. #     def __init__(self, x):
  4. #         self.val = x
  5. #         self.next = None
  7. class Solution:
  8.     def detectCycle(self, head: ListNode) -> ListNode:
  9.         slow = fast = head
  11.         while fast and fast.next:
  12.             slow = slow.next
  13.             fast = fast.next.next
  15.             if slow == fast:
  16.                 fast = head
  17.                 while slow != fast:
  18.                     fast = fast.next
  19.                     slow = slow.next
  20.                 return fast
  22.         return None