linked-list-random-node

https://leetcode.com/problems/linked-list-random-node/

// Using Reservoir sampling algorithm
// https://discuss.leetcode.com/topic/53812/using-reservoir-sampling-o-1-space-o-n-time-complexity-c/2 
/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
ListNode h;
java.util.Random rand;

/** @param head The linked list's head.
Note that the head is guaranteed to be not null, so it contains at least one node. */
public Solution(ListNode head) {
h = head;
rand = new java.util.Random();
}

/** Returns a random node's value. */
public int getRandom() {
if (h == null) {
return 0;
}

int ret = h.val;
int i = 1;
ListNode cur = h;
while (cur.next != null) {
cur = cur.next;
i++;
int f = rand.nextInt() % i;
if (f == 0) {
ret = cur.val;
}
}
return ret;
}
}

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