2021-luvut

input/code.cpp: In function 'bool is_valid(long long int, int*)':
input/code.cpp:11:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int ci = 0; ci < ns.length(); ci++) {
                      ~~~^~~~~~~~~~~~~
input/code.cpp: In function 'int main()':
input/code.cpp:32:25: error: 'indices' was not declared in this scope
         if (is_valid(i, indices)) {
                         ^~~~~~~
input/code.cpp:36:36: error: 'indices' was not declared in this scope
         if (cn == n && is_valid(i, indices)) {
                                    ^~~~~~~

#include <iostream>
#include <string>
#include <cmath>

using namespace std;

bool is_valid(long long num, int* indices) {
    char checklist[] = { '2', '0', '2', '1' };
    int index = 0;
    string ns = to_string(num);
    for (int ci = 0; ci < ns.length(); ci++) {
        if (index == 4) return true;
        if (ns[ci] == checklist[index]) {
            indices[index] = ci;
            index++;
        }
            
    }
    if (index == 4) return true;
    else            return false;
}

int main() {    
    long long n; // the n:th to be found
    cin >> n;
    
    long long cn = 0; // current nth
    
    long long output = 0; // the - output..
    
    for (long long i = 2021; i < pow(10, 12); i++) {
        if (is_valid(i, indices)) {
            cn++;
            //cout <<cn << " : " << i << endl;
        }
        if (cn == n && is_valid(i, indices)) {
            output = i;
            break;
        }
    }
    
    cout << output;
    
    return 0;
}