linked list cycle ii

# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class (object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head:
            return None
        slow = head
        fast = head
        found = False
        counter = 0
        while fast and fast.next:
            if counter != 0:
                if slow is fast:
                    found = True
                    break
            slow = slow.next
            fast = fast.next.next
            counter += 1
        
        if found:
            slow = head
            while slow and fast:
                if slow is fast:
                    return slow
                else:
                    slow = slow.next
                    fast = fast.next
        else:
            return None