Linked List Random Node

Given a singly linked list, return a random node's value from the linked list. Each node must have thesame 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 ：

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *p;    /** @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) {        p = head;    }       /** Returns a random node's value. */    int getRandom() {        ListNode *tem = p;        int val = p->val;        for(int i =1; tem; i++)        {            if(rand()%i==0) val = tem->val;            tem = tem->next;        }        return val;    }};

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