leetcode382. 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();

    private ListNode head;
    private Random r;
    /** @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;
        this.r = new Random();
    }

    /** Returns a random node's value. */
    public int getRandom() {
        ListNode tmp = this.head;
        int result = 0;
        int index = 1;
        do{
            if(r.nextInt(index) == 0) {
                result = tmp.val;
            }
            tmp = tmp.next;
            index++;
        }while(tmp != null);
        return result;
    }