程序占用太多时间

Question

I'm trying to solve a coding problem, the problem is:

• Take a string input
• Take a number input 'n'
• Repeat the string up to n indexes
• Count the number of 'a' characters that occur in the repeated string

This problem was authored by tunyash on Hackerrank with title 'Repeated String'

My current solution is taking too much time to run

This is what I am currently doing:

1. Use a variable to iterate through the original string
2. Each time the variable exceeds the original string length, reset it to 0
3. Iterate n times

I've made a function to do the counting as follows:

long long repeatedString(std::string s, long long n) {
    long long sIndex{ 0 }, length = s.size(), result{ 0 };
    

    for (long long i = 0; i < n; i++)
    {

        if (sIndex > (length - 1))
            sIndex = 0;

        if (s[sIndex] == 'a')
            result += 1;

        sIndex += 1;
    }
    return result;
}

I've tried modifying and using binary search algorithm by first writing the whole string then searching but the writing part takes too much time and seems not very intuitive

Answer 1

This is a typical beginner programming exercise. The idea is that you shouldn't blindly overengineer the problem, when a simple mathematical formula is right around the corner. In this case you can simply count the number of a 's in the original string and multiply it by n to get the desired result:

std::count(s.begin(), s.end(), 'a') * n

where s is your input string.

Edit: I misinterpreted the question. I assumed n was the number of repetitions of the whole string, whereas it actually was the number of characters to concatenate by modularly concatenating the strings characters up until n number of characters. In this case, simply divide before multiplying: n / s.length() and adjust for the n % s.length() characters remaining with addition. I will leave this as an exercise.