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

Python Solution with explanation

The idea is to hold an auxiliary array per nucleotide X, with position i (ignoring zero) is how many times X has occurred as of now. And so if we need the number of occurrences of X from position f to position t, we could take the following equation:

aux(t) - aux(f)

Time complexity is:

O(N+M)

def solution(S, P, Q):
    n = len(S)
    m = len(P)

    aux = [[0 for i in range(n+1)] for i in [0,1,2]]

    for i,c in enumerate(S):
        aux[0][i+1] = aux[0][i] + ( c == 'A' )
        aux[1][i+1] = aux[1][i] + ( c == 'C' )
        aux[2][i+1] = aux[2][i] + ( c == 'G' )

    result = []

    for i in range(m):
        fromIndex , toIndex = P[i] , Q[i] +1
        if   aux[0][toIndex] - aux[0][fromIndex] > 0:
            r = 1
        elif aux[1][toIndex] - aux[1][fromIndex] > 0:
            r = 2
        elif aux[2][toIndex] - aux[2][fromIndex] > 0:
            r = 3
        else:
            r = 4

        result.append(r)

    return result