LeetCode: Linked List Random Node

这题参照http://blog.jobbole.com/42550/

用的蓄水池算法，即更改ans的概率为1/（当前length)

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

     private 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 p = head;
         int ans = p.val;
         for (int i = 1; p.next != null; i++) {
             p = p.next;
             if ((int)(Math.random() * (i + 1)) == i)
                 ans = p.val;
         }
         return ans;
     }
 }

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