382. Linked List Random Node

Problem:

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?

Example:

// Init a singly linked list [1,2,3].
ListNode head = new ListNode(1);
head.next = new ListNode(2);
head.next.next = new ListNode(3);
Solution solution = new Solution(head);

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

Analysis:
实在是看不懂reservoir sampling, 看了2个小时稍微有点儿眉目。。。意思就是随机取一个范围1到i的数，可以保证概率随机。。。。

Solution:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */ class Solution { Random rand; ListNode head; /** @param head The linked list's head. Note that the head is guaranteed to be not null, so it contains at least one node. */ public Solution(ListNode head) { rand = new Random(); this.head = head; } /** Returns a random node's value. */ public int getRandom() { ListNode p = head; int res = p.val; for (int i = 1; p.next != null; i++) { p = p.next; if (rand.nextInt(i + 1) == i) res = p.val; } return res; } } /** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(head); * int param_1 = obj.getRandom(); */