142. 环形链表 II

代码一：快慢指针

class Solution(object):
    # 快慢指针
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow, fast = head, head
        finder = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if fast == slow:
                while finder != slow:
                    finder = finder.next
                    slow = slow.next
                return finder
        return None

代码二：list

class Solution(object):
    # list
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        mylist = []
        while head:
            if head in mylist:
                return head
            else:
                mylist.append(head)
            head = head.next
        return None