!7

p {
  new TreeSet<S> set;
  set.add("pot");
  set.add("potash");
  System.out.println(containsPrefixOf(set, "pot"));
}

sbool containsPrefixOf(TreeSet<S> set, S s) {
  if (set == null || s == null) false;
  while true {
    S key = set.floor(s);
    
    if (key == null) false; // s is in front of whole set => no prefix in there
    int n = lengthOfCommonPrefix(key, s);
    if (n == key.length()) return true; // found!
    s = s.substring(0, n); // shorten and try again
  }
}

static int lengthOfCommonPrefix(String a, String b) {
  int i = 0, n = Math.min(a.length(), b.length());
  while (i < n && a.charAt(i) == b.charAt(i))
    ++i;
  return i;
}