34. 链表中环的入口结点

class Solution(object):
    def entryNodeOfLoop(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow, fast = head, head
        while fast:
            slow, fast = slow.next, fast.next
            if fast: fast = fast.next
            if slow == fast:
                slow = head
                while fast and slow != fast:
                    slow, fast = slow.next, fast.next
                return fast
        return None