Linked List Random Node -- LeetCode

[Question]

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]
Another Reservior Sampling problem. The run time is at O(N) without additional space expense. See also the similar question Random Pick Index.

[Solution]
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
    ListNode* head;
public:
    /** @param head The linked list's head.
        Note that the head is guaranteed to be not null, so it contains at least one node. */
    Solution(ListNode* head): head(head) {
    }
 
    /** Returns a random node's value. */
    int getRandom() {
        int res;
         for (int cnt=1,  ListNode* p = head;  p;  p=p->next, cnt++)
              if (rand()%cnt == 0) res = p->val;
     
        return res;
    }

};