382.Linked List Random Node
===

###### tags: `Medium`,`Linked List`,`Math`

[382. Linked List Random Node](https://leetcode.com/problems/linked-list-random-node/)

### 題目描述

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.

Implement the `Solution` class:

* `Solution(ListNode head)` Initializes the object with the head of the singly-linked list `head`.
* `int getRandom()` Chooses a node randomly from the list and returns its value. All the nodes of the list should be equally likely to be chosen.

### 範例

**Example 1:**


Input
["Solution", "getRandom", "getRandom", "getRandom", "getRandom", "getRandom"]
[[[1, 2, 3]], [], [], [], [], []]
Output
[null, 1, 3, 2, 2, 3]

Explanation
Solution solution = new Solution([1, 2, 3]);
solution.getRandom(); // return 1
solution.getRandom(); // return 3
solution.getRandom(); // return 2
solution.getRandom(); // return 2
solution.getRandom(); // return 3
// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.

**Constraints**:

* The number of nodes in the linked list will be in the range [1, 10^4^].
* -10^4^ <= `Node.val` <= 10^4^
* At most 10^4^ calls will be made to `getRandom`.

**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?

### 解答

#### C++
##### 思路:
第一種: 先遍歷一次得整體數量n再以此為區間隨機取得數字
第二種: 對於無法先遍歷得整體數量n如streaming data需用[Reservoir sampling](https://www.youtube.com/watch?v=aMhe_Riny5E)

class Solution {
public:
    ListNode *head;
    Solution(ListNode* head) {
        this->head = head;
    }
    
    int getRandom() {
        int n = 1, res = 0;
        ListNode* cur = head;
        while(cur)
        {
            int j = rand() % n;
            if(j == 0)
                res = cur->val;
            n++;
            cur = cur->next;
        }
        return res;
    }
};

Time: $O(n)$
Extra Space: $O(1)$

> [name=XD] [time= March 10, 2023]

**Follow up:**
1. 如果選取k個如何做?
2. 如果有m台機器做分散式處理, 全部加總共n個, 選取k個如何做?

### Reference

[回到題目列表](https://hackmd.io/@Marsgoat/leetcode_every_day)