382. Linked List Random Node

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

// 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();

S: reservoir sampling

假设当前为第n个node，保证该node入选的概率为1/n即可

 class Solution {
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) {
        myHead = head;
    }

    /** Returns a random node's value. */
    int getRandom() {
        int count = 0, result = -1;
        ListNode* node = myHead;
        while (node) {
            if (rand() % (count + 1) == 0) {
                result = node->val;
            }
            node = node->next;
            count++;
        }
        return result;
    }
private:
    ListNode *myHead;
};