LeetCode 382: Linked List Random Node

Resevior Sampling : https://en.wikipedia.org/wiki/Reservoir_sampling

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    private ListNode head;
    private Random rand;
    /** @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) {
        this.head = head;
        rand = new Random();
    }
    
    /** Returns a random node's value. */
    public int getRandom() {
        ListNode runner = head.next;
        int result = head.val;
        for (int i = 1; runner != null; i++, runner = runner.next) {
            if (rand.nextInt(i + 1) == i) result = runner.val;
        }
        return result;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(head);
 * int param_1 = obj.getRandom();
 */