class Solution {
    public String nextClosestTime(String time) {
        TreeSet<Character> ts = new TreeSet<>();
        char[] s = time.toCharArray();
        for (char ch : s) {
          if (Character.isDigit(ch)) {
            ts.add(ch);
          }
        }

        if (ts.size() < 2 || ts.size() > 4) {
          return "Not possible / Invalid argument";
        }

        for (int i = s.length-1; i >= 0; i--) {
          if (!Character.isDigit(s[i]) || ts.higher(s[i]) == null) {
            continue;
          }

          s[i] = ts.higher(s[i]);
          for (int j = i+1; j < s.length; j++) {
            if (!Character.isDigit(s[j])) continue;
            s[j] = ts.first();
          }
          return new String(s);
        }
        return new String(s);

    }
}