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();

Solution

class Solution {

    /** @param head The linked list's head.
        Note that the head is guaranteed to be not null, so it contains at least one node. */
    ListNode head;
    Random random;
    public Solution(ListNode head) {
        this.head = head;
        random = new Random();
    }
    
    /** Returns a random node's value. */
    public int getRandom() {
        ListNode cur = head;
        int res = cur.val, count = 1;
        while (cur.next != null) {
            cur = cur.next;
            if (random.nextInt(count+1) == count) {
                res = cur.val;
            }
            count++;
        }
        return res;
    }
}