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();
 */