142. 环形链表 II

题目链接	142. 环形链表 II
思路	快慢指针-经典应用：相遇后，移动head及slow直至相遇
题解链接	没想明白？一个视频讲透！含数学推导（Python/Java/C++/Go/JS）
关键点	while fast and fast.next: ... && while head != slow: ...
时间复杂度	\(O(n)\)
空间复杂度	\(O(1)\)

代码实现：

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 fast is slow:
                while slow is not head:
                    slow = slow.next
                    head = head.next
                return head
        return None