Code for DNA sequence in codility

This idea is implemented by calculating prefix sum first to record the number of appearance of nucleotides in a target string. After that, for each query, it will check whether a nucleotide is a minimum factor or not.

def solution(S, P, Q):
    records = [[0]*4 for i in range(len(S))]
    results = [0] * len(P)
    
    for i in range(len(S)):
        if (S[i] == 'A'):
            records[i][0] = 1
        elif (S[i] == 'C'):
            records[i][1] = 1
        elif (S[i] == 'G'):
            records[i][2] = 1
        elif (S[i] == 'T'):
            records[i][3] = 1
            
    for i in range(1, len(S)):
        for j in range(4):
            records[i][j] += records[i-1][j]
    
    for i in range(len(P)):
        head = P[i]
        tail = Q[i]
        
        for j in range(4):
            sub = 0
            if (head - 1 >= 0):
                sub = records[head-1][j]
            if (records[tail][j] - sub > 0):
                results[i] += j + 1
                break
    
    return results