[leetcode #382] Linked List Random Node

Problem

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 integer array nums.
・ 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 choosen.

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, 10⁴].
・ -10⁴ <= Node.val <= 10⁴
・ At most 10⁴ 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?

Idea

리스트에 있는 값 중 랜덤으로 하나를 선택하는 문제다.

리스트 중 하나를 랜덤으로 선택하는 건 비효율적이므로, 리스트를 탐색하면서 ArrayList에 따로 저장했다. 이후 Randam 클래스의 nextInt에 0~(리스트의 크기) 사이의 값을 index로 선택하여 ArrayList의 값을 리턴하면 된다.

Solution

/**
 * 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 {
    List<Integer> list = new ArrayList<>();
    Random random = new Random();

    public Solution(ListNode head) {
        ListNode node = head;
        while (node != null) {
            list.add(node.val);
            node = node.next;
        }
    }

    public int getRandom() {
        return list.get(random.nextInt(list.size()));
    }
}

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

Reference

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