Monistus

input/code.cpp: In function 'std::string expand_recurse(std::string_view)':
input/code.cpp:28:71: warning: comparison of integer expressions of different signedness: 'std::basic_string_view<char>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   28 |             if ('1' <= sub[i] && sub[i] <= '9' && (sub.size() - i - 1 < sub[i] - '0')) {
      |                                                    ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

#include <iostream>
#include <sstream>
#include <string>
#include <vector>
 
constexpr char const* digits = "123456789";
 
///
/// \brief expand_recurse Expand the sting from the next leading number, recursively
/// \param str
/// \return expanded result
/// First recursively calculates the expanded form of the n following characters, then duplicates it
/// twice
///
std::string expand_recurse(std::string_view str) {
    size_t next_digit;
    size_t pos = 0;
 
    std::string result(str);
 
    while (next_digit = result.find_first_of(digits, pos), next_digit != result.npos) {
        pos            = next_digit;
        size_t const n = result[next_digit] - '0';
        auto sub       = std::string_view(result).substr(next_digit + 1, n);
 
        bool expand = false;
        for (size_t i = 0; i < sub.size(); ++i) {
            if ('1' <= sub[i] && sub[i] <= '9' && (sub.size() - i - 1 < sub[i] - '0')) {
                expand = true;
                break;
            }
        }
        std::string next;
 
        if (expand) {
            next = static_cast<std::string>(sub);
        } else {
            // Recursively expand
            next = expand_recurse(sub);
        }
        next += next;
 
        result.replace(next_digit, 1 + n, next, 0, 1 + n);
        result.insert(next_digit + 1 + n, next, 1 + n);
    }
 
    return result;
}
 
int main() {
 
//    std::istringstream input("mis3si1pi");
    auto& input = std::cin;
 
    std::string str;
    std::getline(input, str);
 
    auto res = expand_recurse(str);
    std::cout << res << "\n";
//    std::cout << res.size() << "\n";
}

Test 1

Group: 1, 2

Verdict: ACCEPTED

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: ACCEPTED

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: ACCEPTED

Test 6

Group: 1, 2

Verdict: ACCEPTED

Test 7

Group: 1, 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 2

Verdict: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: ACCEPTED