LeetCode 142的解法,求求帮看哪里错了

# 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:
        if not head:
            return None
        slow,fast = head,head
        mixed= None
        while fast and fast != slow:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                slow = head
                while fast != slow:
                    fast = fast.next
                    slow = slow.next
                mixed = slow
                break

        if not mixed:
            return None
        else:
            i = 0
            while head!=mixed:
                head = head.next
                i+=1
            return i