public Map<String, String> pairs(String[] strings) {

    Map<String, String> map = new HashMap<String, String>();

    if (strings != null){
      for (String s:strings) {
        int n = s.length();
        if (!map.containsKey(s)) {  // first time we've seen this string
          map.put(s.substring(0,1), s.substring(n-1, n));
        }
  }


    }


    return map;
}