exercises in java

exercises.java

public class MyClass {
    public static void main(String args[]) {
      
        getUnicLetter("esta es una cadena con letras repetidas","");
    }
    
    public static String getUnicLetter(String originalWord,String finalWord){
        if(originalWord.isEmpty()) return  null;
        System.out.print(finalWord);
        return getUnicLetter(originalWord.replace(originalWord.substring(0,1),""),finalWord.contains(originalWord.substring(0,1)) ? "" : originalWord.substring(0,1));
    }
    
}

i need change this solution, becase i cant use the substring because create a new string constantly :(