Using stack DS

static StringBuilder res = new StringBuilder(); static StackstringStack = new Stack<>(); public static void main(String[] args) { Scanner scanner = new Scanner(System.in); int q = scanner.nextInt();

    //number of quiries
    while( (q--) > 0 ){
        int typeOfOp= scanner.nextInt();
        switch (typeOfOp){
            case 1:
                String stringToAppend = scanner.next();
                append(stringToAppend);
                break;
            case 2:
                int k = scanner.nextInt();
                deleteLast_K_char(k);
                break;

            case 3:
                int kToPrint = scanner.nextInt();
                printKchar(kToPrint);
                break;

            case 4:
                undo(); 

        }

    }
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
}


public static void append(String s){
    stringStack.push(res.toString());
    res.append(s);
}

public static void deleteLast_K_char(int k){
     stringStack.push(res.toString());
     //System.out.println("before delete ----> " + res);
     res.delete(res.length() - k, res.length());

    //System.out.println("after delete ----> "+ res);
}

public static void printKchar(int k){
    System.out.println(res.charAt(k-1));
}

public static void undo(){
    res=new StringBuilder(stringStack.pop());

}