LeetCode in Kotlin

382. Linked List Random Node

Medium

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?

Solution

import com_github_leetcode.ListNode
import kotlin.random.Random

/*
 * Example:
 * var li = ListNode(5)
 * var v = li.`val`
 * Definition for singly-linked list.
 * class ListNode(var `val`: Int) {
 *     var next: ListNode? = null
 * }
 */
@Suppress("NAME_SHADOWING", "kotlin:S2245")
class Solution(head: ListNode?) {
    private val al: MutableList<Int>

    init {
        var head = head
        al = ArrayList()
        while (head != null) {
            al.add(head.`val`)
            head = head.next
        }
    }

    fun getRandom(): Int {
        /*
        Math.random() will generate a random number b/w 0 & 1.
        then multiply it with the array size.
        take only the integer part which is a random index.
        return the element at that random index.
         */
        val ind = Random.nextInt(al.size)
        return al[ind]
    }
}

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

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