382. Linked List Random Node**

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].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();

My code:

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    private Random random;    int len;    ListNode head;    /** @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) {        this.head = head;    }        /** Returns a random node's value. */    public int getRandom() {        ListNode temp = head;        len = 0;        while(temp!=null){            len++;            temp=temp.next;        }        random = new Random();        int r = random.nextInt(len);        temp=head;        for(int i=0;i<r;i++){            temp = temp.next;        }        return temp.val;    }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(head); * int param_1 = obj.getRandom(); */