# 382. Linked List Random Node (Medium)

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.

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();
```

## Solutions

``````/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
class Solution {
Random random;

Note that the head is guaranteed to be not null, so it contains at least one node. */
public Solution(ListNode head) {
this.random = new Random();
}

/** Returns a random node's value. */
public int getRandom() {
ListNode curr = head;
ListNode rst = null;
for (int i = 1; curr != null; i++) {
// random.nextInt(n) between 0 (inclusive) and n (exclusive)
// determine whether the ith is put into the pool or not.
// For the nth target, i is n.
// Then the probability that rnd.nextInt(i)==0 is 1/n.
// Thus, the probability that return nth target is 1/n.
if (random.nextInt(i) == 0) {
rst = curr;
}
curr = curr.next;
}
return rst.val;
}
}

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