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();
 */