public static int stringConstruction(String s) {
  boolean[] freqs = new boolean[26]; // a to z
  int cost = 0;
  for (char c : s.toCharArray()) {
    // If the character hasn't been added before, increase the cost
    if (!freqs[c - 'a']) {
      freqs[c - 'a'] = true;
      cost++;
    }
  }
  return cost;
}