Optimization Using Precalculation in Go

Welcome to this exciting analysis lesson! In this unit, we're going to take a practical approach and introduce a technique called Optimization Using Precalculation. 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!

Our task here involves a slice 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 slice. Your goal is to write a Go function that, for each query, returns the minimum value in the slice 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. The idea here is to pre-calculate the minimum value for each possible l and r, store these values, and then proceed with the queries. This way, we can simplify the problem and enhance the speed of our solution by eliminating redundant computations.

The function will accept three parameters: arr, Ls, and Rs. The primary slice is arr, while Ls and Rs are slices that hold the l and r values, respectively, for each query. For instance, let's say you have a slice like {2, 1, 3, 7, 5} and the following queries: {0, 2, 4} and {1, 3, 4}. The aim of our function would be to return a slice {1, 3, 5} as the minimum values within the ranges of the three queries.

We'll begin by declaring our function QueryMin.

Initially, we'll set n to be the size of the given slice and create a two-dimensional slice, precalc, filled with zeros. This slice will store all the pre-calculated minimums for all possible range pairs (l, r).

package main

import (
    "fmt"
    "math"
)

func QueryMin(arr []int, Ls []int, Rs []int) []int {
    n := len(arr)
    precalc := make([][]int, n)
    for i := range precalc {
        precalc[i] = make([]int, n)
    }

Now, we shall pre-calculate the minimum value for all possible l and r pairs. This is essentially the brute-force approach, but by doing this upfront, we optimize 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 slice.

    for l := 0; l < n; l++ {
        minVal := arr[l]
        for r := l; r < n; r++ {
            if arr[r] < minVal {
                minVal = arr[r]
            }
            precalc[l][r] = minVal
        }
    }