# 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();

## Thoughts

满足Uniform distribution的随机采链表节点值，要求O(1)额外空间。O(1)空间采样=>蓄水池，遍历并根据当前长度做采样，当采到上次采过的元素则做替换。

## Code

```cpp
/*
 * @lc app=leetcode id=382 lang=cpp
 *
 * [382] Linked List Random Node
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    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. */
    Solution(ListNode* head) {
        head_ = head;
    }
    
    /** Returns a random node's value. */
    int getRandom() {
        int res = -1;
        auto cur = head_;
        for (int cnt = 1; cur != nullptr; cur = cur->next, ++cnt) {
            if (rand() % cnt == 0) res = cur->val;
        }
        return res;
    }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(head);
 * int param_1 = obj->getRandom();
 */
// @lc code=end


```

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    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() {
        int res = -1;
        Random rand = new Random();
        int count = 0;
        ListNode node = head;
        while (node != null) {
            if (rand.nextInt(++count) == 0) {
                res = node.val;
            }
            node = node.next;
        }
        return res;
    }
}

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

## Analysis

每次getRandom 需要O(N).


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://hao-fu-1.gitbook.io/oj/math/reservoir-sampling/linked-list-random-node.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.