Cycle Ii

LeetCode Problem

# https://leetcode.com/problems/linked-list-cycle-ii/
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:
                break
        else:
            return None
        # cycle found
        ptr1 = head
        ptr2 = slow
        while ptr1 != ptr2:
            ptr1 = ptr1.next
            ptr2 = ptr2.next
        return ptr1