f-task/main.cpp

#include <cctype>
#include <cstdlib>
#include <iostream>
#include <string>

std::string decode(std::string& strIn, std::string::iterator& itr, bool firstCircle = false)
{
    std::string out;

    while (itr != strIn.end())
    {
        std::string strNum;
        auto repeat = 1;

        if (std::isdigit(static_cast<unsigned>(*itr)))
        {
            while (*itr != '[')
            {
                strNum += *itr++;
            }
            repeat = std::atoi(strNum.c_str());
            auto rptOut = decode(strIn, ++itr);

            for (int i = 0; i < repeat; ++i)
                out += rptOut;
        }

        if (*itr == ']')
        {
            if (!firstCircle)
                return out;
        }
        else
            out += *itr;

        ++itr;
    }

    return out;
}

std::string kek(std::string strIn)
{
    std::cout << strIn << std::endl;
    auto itr = strIn.begin();

    return decode(strIn, itr, true);
}

void compare(std::string expected, std::string out)
{
    if (expected == out)
        std::cout << "ok";
    else
        std::cout << "not ok. expected: " << expected << std::endl
                  << "             out: " << out;

    std::cout << std::endl;
}

int main(int argc, char* argv[])
{
    std::string strIn, strOut;

    strIn = "3[a2[c]]";
    strOut = "accaccacc";
    // std::cout << kek(strIn) << std::endl;
    compare(strOut, kek(strIn));

    strIn = "c2[a]3[b]c";
    strOut = "caabbbc";
    // std::cout << kek(strIn) << std::endl;
    compare(strOut, kek(strIn));

    strIn = "c2[a3[b]]c";
    strOut = "cabbbabbbc";
    // std::cout << kek(strIn) << std::endl;
    compare(strOut, kek(strIn));

    strIn = "231[ab]";
    strOut = "ababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababab";
    // std::cout << kek(strIn) << std::endl;
    compare(strOut, kek(strIn));

    strIn = "3[b15[a]]";
    strOut = "baaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaa";
    // std::cout << kek(strIn) << std::endl;
    compare(strOut, kek(strIn));

    return 0;
}