LeetCode 382 Linked List Random Node (蓄水池算法 推荐)

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();

题目链接：https://leetcode.com/problems/linked-list-random-node/

题目分析：蓄水池算法 经典的等概率随机算法

/**
* 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. */

ListNode head;
Random rand;

public Solution(ListNode head) {
this.head = head;
}

/** Returns a random node's value. */
public int getRandom() {
ListNode ans = head;
ListNode cur = head;
int cnt = 0;
rand = new Random();
while (cur != null) {
if (rand.nextInt(cnt + 1) == cnt) {
ans = cur;
}
cur = cur.next;
cnt ++;
}
return ans.val;
}
}

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