Day7 . 142.环形链表II

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

核心思想：快慢指针 绕圈

定义快指针走2步，慢指针走1步，快慢指针相遇

#
# @lc app=leetcode.cn id=142 lang=python3
#
# [142] 环形链表 II
#

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

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        slow = fast = head

        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

            if slow == fast:
                slow = head

                while slow != fast:
                    slow = slow.next
                    fast = fast.next
                return slow
        return None
# @lc code=end