# LeetCode – Linked List Random Node (Java)

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?

**Java Solution**

This problem is trivial, so I focus on the follow-up problem.

We want the probability of being chosen is 1/count.

Given a list 1 -> 2 -> 3 -> 4 -> 5 -> ... -> n, the only time we can possibly select the third node is when the pointer points to 3. The probability of selecting the third node is 1/3 * 3/4 * 4/5 * ... * (n-1)/n = 1/n.

public class Solution { /** @param head The linked list's head. Note that the head is guanranteed to be not null, so it contains at least one node. */ Random r=null; ListNode h=null; public Solution(ListNode head) { r = new Random(); h = head; } /** Returns a random node's value. */ public int getRandom() { int count=1; ListNode p = h; int result = 0; while(p!=null){ if(r.nextInt(count)==0) result= p.val; count++; p = p.next; } return result; } } |

<pre><code> String foo = "bar"; </code></pre>