蓄水池算法（链表随机节点）

Posted 2021-09-03 秦枫-_-

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    private ListNode head;
    public Solution(ListNode head) {
        this.head = head;
    }
    public int getRandom() {
        //直接假设袋子容量为1
        int count = 1;
        int res = head.val;
        ListNode temp = head;
        temp=temp.next;
        while (temp != null) {
            count++;
         if (rand(count) == 1) {//第 i 个数被替换(cur进入蓄水池)的概率为1/count，前面第i个数不被替换(cur不进入蓄水池)的概率为（count-1）/count，如此可以做到每个数被保留的概率为m/N
                    res = temp.val;
                } 
            temp = temp.next;
        }
        return res;
    }
    public int rand(int max){
        return (int)(Math.random()*max)+1;
    }
}