public static void main(String args[]){
        int input[] = {2,3,1,5};
        KeyPadPermutation kpp = new KeyPadPermutation();
        kpp.permute(input);
    }
http://stackoverflow.com/questions/2344496/how-can-i-print-out-all-possible-letter-combinations-a-given-phone-number-can-re
src/com/interview/recursion/KeyPadPermutation.java