code: public Trie lookup(CharSequence s) {
      Trie t = this;
      for (int i = 0, n = s.length(); i < n; ++i) {
        t = t.lookup(s.charAt(i));
        if (null == t) { break; }
      }
      return t;
    }
docs: The descendant of this trie corresponding to the string for this trie
  appended with s.
  @param s non null.
  @return null if no such trie.