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();
代码如下：（方法一）

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) { val = x; }
  * }
  */
 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. */

     private List<Integer> list=new ArrayList<>();

     public Solution(ListNode head) {
         ListNode p=head;
         int i=0;
         while(p!=null)
         {
         list.add(p.val);
         p=p.next;
         i++;
         }
     }

     /** Returns a random node's value. */
     public int getRandom() {
         Random random=new Random();

         int q=random.nextInt(list.size());
         return list.get(q);

     }
 }

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

方法二：（参考别人的）

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) { val = x; }
  * }
  */
 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. */

     private ListNode list=null;
     private int size=0;
     public Solution(ListNode head) {
         list=head;
         ListNode p=head;
         while(p!=null)
         {
         p=p.next;
         size++;
         }
      }

     /** Returns a random node's value. */
     public int getRandom() {
         ListNode ss=list;
         int rand=(int)(Math.random()*size);
         while(rand>0&&ss!=null)
         {
             ss=ss.next;
             rand--;
         }
         return ss!=null?ss.val:0;
     }
 }

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