LeetCode: Linked List Random Node

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10 
11     private ListNode head;
12     /** @param head The linked list‘s head.
13         Note that the head is guaranteed to be not null, so it contains at least one node. */
14     public Solution(ListNode head) {
15         this.head = head;
16     }
17     
18     /** Returns a random node‘s value. */
19     public int getRandom() {
20         ListNode p = head;
21         int ans = p.val;
22         for (int i = 1; p.next != null; i++) {
23             p = p.next;
24             if ((int)(Math.random() * (i + 1)) == i) 
25                 ans = p.val;
26         }
27         return ans;
28     }
29 }
30 
31 /**
32  * Your Solution object will be instantiated and called as such:
33  * Solution obj = new Solution(head);
34  * int param_1 = obj.getRandom();
35  */