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:

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// 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();
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也是一个蓄水池抽样算法(Reservoir Sampling)的题目，跟398. Random Pick Index基本一样，就不多说了，具体算法可参考398题。

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