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

    /** @param
    private ListNode list=null;
    private int size=0;
    public Solution(ListNode head) {
        list=head;
        ListNode p=head;
        while(p!=null)
        {
            p=p.next;
            size++;
        }
    }

    /** Returns a random node's value. */
    public int getRandom() {
        ListNode ss=list;
        int rand=(int)(Math.random()*size);
        while(rand>0&&ss!=null)
        {
            ss=ss.next;
            rand--;
        }
        return ss!=null?ss.val:0;
    }
}

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