revc.cpp

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

// reverse complement of a DNA string
int main()
{
    string s;
    cin >> s; // input from stdin
    char n;

    // loop through s in reverse order
    for (int i = s.length(); i >=0; --i) {
    	// get current nucleotide as uppercase
	n = toupper(s[i]);

	// get complement
	if (n == 'A') {
	    n = 'T';
	} else if (n == 'T') {
	    n = 'A';
	} else if (n == 'C') {
	    n = 'G';
	} else if (n == 'G') {
	    n = 'C';
	}
	cout << n; // send to stdout
    }
    cout << endl;
    return 0;
}