Efficient Array Query Optimization with Precalculation in Kotlin | CodeSignal Learn

Efficient Array Query Optimization with Precalculation in Kotlin | CodeSignal Learn

Introduction

Welcome to this exciting analysis lesson! In this unit, we're going to take a practical approach and utilize several intriguing techniques, such as Skipping Redundant Cases, Optimization Using Precalculation, and Picking the Best Variable to Iterate Over . Our platform for this unit is an array-based problem, where we'll apply these techniques to formulate an efficient and optimized solution. Ready for the challenge? Then, let's get started!

Task Statement

Our task here involves an array composed of at most 1,000 elements and potentially millions of queries. Each query is a pair of integers denoted as l and r, which correspond to some indices in the array. Your goal is to write a method that, for each query, returns the minimum value in the array between indices l and r (inclusive). The catch is this: rather than directly finding the minimum value for each query one by one, we're required to optimize the process. We will precalculate the minimum value for each possible l and r interval, store these values, and then use them to efficiently answer each query. This approach simplifies the problem and enhances the speed of our solution by eliminating redundant computations. The method will accept three parameters: arr, Ls, and Rs . The primary array is arr, while Ls and Rs are MutableList instances that hold the l and r values, respectively, for each query. For instance, let's say you have an array like [2, 1, 3, 7, 5] and the following queries: [0, 2, 4] and [1, 3, 4] . The aim of our method would be to return [1, 3, 5] as the minimum values within the ranges of the three queries.

Solution Building: Step 1

We'll begin by declaring our function queryMin . Initially, we'll set n to be the size of the given array and create a two-dimensional array precalc, filled with zeroes. This array will store all the precalculated minimums for all possible range pairs (l, r). Kotlin fun queryMin(arr: IntArray, Ls: MutableList<Int>, Rs: MutableList<Int>): List<Int> {val n = arr.size val precalc = Array(n) { IntArray(n) }

Solution Building: Step 2

Now, we shall precalculate the minimum value for all possible l and r pairs. Although this is essentially a brute force approach, doing this upfront optimizes our subsequent queries. We loop through every possible range within arr, updating the minimum value found for that range, and store this minimum value in our precalc two-dimensional array. Kotlin for (l in 0 until n) {var minVal = arr[l] for (r in l until n) {minVal = minOf(minVal, arr[r]) precalc[l][r] = minVal}}

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Solution Building: Step 3

With our precalculation complete, we're ready to process the actual queries (the pairs of l and r). For each query, we use the precalculated values in precalc to quickly find the minimum value for that query's range. These values are then added to our result list res . val res = mutableListOf<Int>() for (i in Ls.indices) {val l = Ls[i] val r = Rs[i] res.add(precalc[l][r])} return res}

Main Function

Let’s test our function with a sample input: fun main() {val arr = intArrayOf(2, 1, 3, 7, 5) val Ls = mutableListOf(0, 2, 4) val Rs = mutableListOf(1, 3, 4) val result = queryMin(arr, Ls, Rs) for (value in result) {println(value)} // Expected output: // 1 // 3 // 5}

Kotlin

fun main() {
    val arr = intArrayOf(2, 1, 3, 7, 5)
    val Ls = mutableListOf(0, 2, 4)
    val Rs = mutableListOf(1, 3, 4)
    
    val result = queryMin(arr, Ls, Rs)
    
    for (value in result) {
        println(value)
    }
    // Expected output:
    // 1
    // 3
    // 5
}