return k random numbers from a large linked list

There is a linked list of numbers of length N. N is very large and you don't know N. You have to write a function that will return k random numbers from the list. Numbers should be completely random.
给你一个长度为N的链表。N很大，但你不知道N有多大。你的任务是从这N个元素中随机取出k个元素。你只能遍历这个链表一次。你的算法必须保证取出的元素恰好有k个，且它们是完全随机的（出现概率均等）。

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