leetcode： Reservoir Sampling：Linked List Random Node（382）

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();

class Solution {
ListNode* hd;
public:
/** @param head The linked list's head. Note that the head is guanranteed to be not null, so it contains at least one node. */
Solution(ListNode* head) : hd(head) {}

/** Returns a random node's value. */
int getRandom() {
ListNode* node = hd;
int cnt = 0, randVal = 0;
while(node) {
if(rand() % (++cnt) == 0)
randVal = node->val;
node = node->next;
}
return randVal;
}
};

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