java codility training Genomic-range-query

Question

The task is:

A non-empty zero-indexed string S is given. String S consists of N characters from the set of upper-case English letters A, C, G, T.

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Answer 1

I implemented a Segment Tree solution in Kotlin

import kotlin.math.*

fun solution(S: String, P: IntArray, Q: IntArray): IntArray {

    val a = IntArray(S.length)
    for (i in S.indices) {
        a[i] = when (S[i]) {
            'A' -> 1
            'C' -> 2
            'G' -> 3
            'T' -> 4
            else -> throw IllegalStateException()
        }
    }

    val segmentTree = IntArray(2*nextPowerOfTwo(S.length)-1)
    constructSegmentTree(a, segmentTree, 0, a.size-1, 0)

    val result = IntArray(P.size)
    for (i in P.indices) {
        result[i] = rangeMinQuery(segmentTree, P[i], Q[i], 0, a.size-1, 0)
    }
    return result
}

fun constructSegmentTree(input: IntArray, segmentTree: IntArray,  low: Int,  high: Int,  pos: Int) {

    if (low == high) {
        segmentTree[pos] = input[low]
        return
    }
    val mid = (low + high)/2
    constructSegmentTree(input, segmentTree, low, mid, 2*pos+1)
    constructSegmentTree(input, segmentTree, mid+1, high, 2*pos+2)
    segmentTree[pos] = min(segmentTree[2*pos+1], segmentTree[2*pos+2])
}

fun rangeMinQuery(segmentTree: IntArray, qlow:Int, qhigh:Int ,low:Int, high:Int, pos:Int): Int {

    if (qlow <= low && qhigh >= high) {
        return segmentTree[pos]
    }
    if (qlow > high || qhigh < low) {
        return Int.MAX_VALUE
    }
    val mid = (low + high)/2
    return min(rangeMinQuery(segmentTree, qlow, qhigh, low, mid, 2*pos+1), rangeMinQuery(segmentTree, qlow, qhigh, mid+1, high, 2*pos+2))
}

fun nextPowerOfTwo(n:Int): Int {
    var count = 0
    var number = n
    if (number > 0 && (number and (number - 1)) == 0) return number
    while (number != 0) {
        number = number shr 1
        count++
    }
    return 1 shl count
}