# 382. Linked List Random Node

### Problem:

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?

Example:

``````// Init a singly linked list [1,2,3].

// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.
solution.getRandom();
``````

### Solutions:

``````/**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
private HashMap<Integer, Integer> indexToNum;
private int size = 0;
private Random r;
Note that the head is guaranteed to be not null, so it contains at least one node. */
indexToNum = new HashMap<Integer, Integer>();
}
r = new Random();
}

/** Returns a random node's value. */
public int getRandom() {
if (size > 0) {
int index = r.nextInt(size);
int val = indexToNum.get(index);
return val;
}
return -1;
}
}

/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(head);
* int param_1 = obj.getRandom();
*/
``````
``````/**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
private Random r=null;
private ListNode h=null;
r = new Random();
}

/** 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;
}
}

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