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

Java, 100/100, but with no cumulative/prefix sums! I stashed the last occurrence index of lower 3 nucelotides in a array "map". Later I check if the last index is between P-Q. If so it returns the nuclotide, if not found, it's the top one (T):

class Solution {

int[][] lastOccurrencesMap;

public int[] solution(String S, int[] P, int[] Q) {
    int N = S.length();
    int M = P.length;

    int[] result = new int[M];
    lastOccurrencesMap = new int[3][N];
    int lastA = -1;
    int lastC = -1;
    int lastG = -1;

    for (int i = 0; i < N; i++) {
        char c = S.charAt(i);

        if (c == 'A') {
            lastA = i;
        } else if (c == 'C') {
            lastC = i;
        } else if (c == 'G') {
            lastG = i;
        }

        lastOccurrencesMap[0][i] = lastA;
        lastOccurrencesMap[1][i] = lastC;
        lastOccurrencesMap[2][i] = lastG;
    }

    for (int i = 0; i < M; i++) {
        int startIndex = P[i];
        int endIndex = Q[i];

        int minimum = 4;
        for (int n = 0; n < 3; n++) {
            int lastOccurence = getLastNucleotideOccurrence(startIndex, endIndex, n);
            if (lastOccurence != 0) {
                minimum = n + 1; 
                break;
            }
        }

        result[i] = minimum;
    }
    return result;
}

int getLastNucleotideOccurrence(int startIndex, int endIndex, int nucleotideIndex) {
    int[] lastOccurrences = lastOccurrencesMap[nucleotideIndex];
    int endValueLastOccurenceIndex = lastOccurrences[endIndex];
    if (endValueLastOccurenceIndex >= startIndex) {
        return nucleotideIndex + 1;
    } else {
        return 0;
    }
}
}