Programming

% perl -MMath::Combinatorics -le'
  @n = qw(A B C);
  print join ",",
    map { join "", @$_ }
  combine 2, @n
  '
AB,AC,BC

#include <iostream>
#include <sstream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <list>
#include <set>

using namespace std;


int main()
{
	string ch[] = {"THIS","IS","OK"};
	vector<string> vec(ch,ch+3);
	set<string> st;

	sort(vec.begin(),vec.end());

	do{
		ostringstream oss;
		vector<string>::iterator pos = vec.begin();	
		oss<<*pos<<*(pos+1);
		if(!st.count(oss.str()))
			st.insert(oss.str());
	}while(next_permutation(vec.begin(),vec.end()));
	
	copy(st.begin(),st.end(),ostream_iterator<string>(cout,"\n"));
}