382. Linked List Random Node (M)

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();

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/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    ListNode head;    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;    }        /** Returns a random node's value. */    public int getRandom() {        r=new Random();        int count=0;        ListNode now=head;        int re=-1;        while(now!=null){            if(r.nextInt(++count)==0)                re=now.val;            now=now.next;            }        return re;        }}/** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(head); * int param_1 = obj.getRandom(); */