382. Linked List Random Node

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();

Reservoir Sampling

import java.util.Random;class Solution {ListNode root;Random random = new Random();    /** @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) {        root = head;    }        /** Returns a random node's value. */    public int getRandom() {        ListNode c = root;        int r = c.val;        for(int i=1; c.next!=null; i++) {        c = c.next;        if(random.nextInt(i+1) == i)r = c.val;        }        return r;    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(head); * int param_1 = obj.getRandom(); */