LeetCode-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();

Analysis:
One way is reservoir sampling: https://en.wikipedia.org/wiki/Reservoir_sampling
Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    ListNode first;

    /** @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) {
        first = head;
    }
    
    /** Returns a random node's value. */
    public int getRandom() {
        ListNode target = first;
        int val = target.val;
        Random engine = new Random();
        for (int i=1;target!=null;i++){
            if (engine.nextInt(i)==0){
                val = target.val;
            }
            target = target.next;
        }
        return val;
    }
}

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

Solution 2: Regular answer

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    Map<Integer,Integer> indexMap;
    int index;

    /** @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) {
        indexMap = new HashMap<Integer,Integer>();
        index = 0;
        ListNode cur = head;
        while (cur!=null){
            indexMap.put(index,cur.val);
            index++;
            cur = cur.next;
        }
    }
    
    /** Returns a random node's value. */
    public int getRandom() {
       Random engine = new Random();
       int ind = engine.nextInt(index);
       return indexMap.get(ind);
    }
}

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