public String conCatShortestFirst(String s1, String s2)

Given TWO Strings, return their concatenation, such that the shorter one comes first.

  int len1 = s1.length();
  int len2 = s2.length();
  String s = s1 + s2; // default: s1 is shorter
  if (len2 < len1) {
    s = s2 + s1;  // if s2 is shorter!
  }
  return s;