边刷leetcode边学编程-382 Linked List Random Node

问题简述

Given a singly linked list, return a random node’s value from the linked list. Each node must have the same probability of being chosen.

Follow up:
What if the linked list is extremely large and its length is unknown to you? Could you solve this efficiently without using extra space?

// Init a singly linked list [1,2,3].

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ListNode head = new ListNode(1);
head.next = new ListNode(2);
head.next.next = new ListNode(3);
Solution solution = new Solution(head);

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// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.
solution.getRandom();

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蓄水池抽样算法

首先储存了k 个数的水池，取每一个数的概率都为1k ,紧接着有第k+1 个数，则需要考虑这个数要不要放进去，放进去水池替换原有的数据的概率为kk+1 。具体要替换的数据的概率为1k 。所以新数据出现的概率为kk+1×1k ，原有的数据的出现的概率是(1−kk+1×1k)×1k 。1.

代码

代码是别人的，(lll￢ω￢)
https://discuss.leetcode.com/topic/53812/using-reservoir-sampling-o-1-space-o-n-time-complexity-c/2

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1. 首先计算该数据不被选择替换出局的概率，然后要计算该数据在剩余的水库中被选中。 ↩