142. 环形链表 II

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None:
            return None
        if head.next is None:
            return None
        first = second = head
        while second.next and second.next.next:
            first = first.next
            second = second.next.next
            if first == second:
                p = head
                while first != p:
                    p = p.next
                    first = first.next
                return p
        return None