#include <string>
#include <iostream>
#include <cassert>
using namespace std;

string complement(string s)
{
    string n;
    string out_string;
    // loop through s in reverse order
    for (int i=s.length(); i>0; i--) {
        // get current nucleotide as uppercase
        n = toupper(s[i]);
        if (n == "A") {
            n = "T";
        } else if (n == "T") {
            n = "A";
        } else if (n == "C") {
            n = "G";
        } else if (n == "G") {
            n = "C";
        }
        out_string += n;
    }
    cout << "input length = " << s.length() << endl;
    cout << "output length = " << out_string.length() << endl;
    assert(s.length() == out_string.length());
    return out_string;
}


int main()
{
	string test_string = "CGTA";
	string out = complement(test_string);
	cout << "input:  " << test_string << endl;
	cout << "output: " << out << endl;
	return 0;
}