```
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
private:
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() {
ListNode *ret = _head;
ListNode *cur = _head->next;
int count = 1;
while (cur != NULL) {
++count;
int rnd = random() % count;
if (rnd == 0)
ret = cur;
cur = cur->next;
}
return ret->val;
}
};
```

Reference: Reservoir sampling

For detailed explanation: My Blog