STE.java

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;


public class Solution {
static class Node<TYPEA, TYPEB>
    {
        final TYPEA action;
        final TYPEB text;

        public Node(final TYPEA vala, final TYPEB valb)
        {
            action = vala;
            text = valb;
        }
    }
 static StringBuffer removeBlankSpace(final StringBuffer sb)
    {
        int j = 0;
        for (int i = 0; i < sb.length(); i++)
        {
            if (!Character.isWhitespace(sb.charAt(i)))
            {
                sb.setCharAt(j++, sb.charAt(i));
            }
        }
        sb.delete(j, sb.length());
        return sb;
    }

    
    public static void main(final String[] args)
    {
        final Stack<Node> myStack = new Stack<Node>();
        StringBuffer strBuf = new StringBuffer();

        final Scanner scan = new Scanner(System.in);
        final int Q = scan.nextInt();

        for (int i = 0; i < Q; i++)
        {

            final int command = scan.nextInt();
            switch (command)
            {
                case 1:
                    final String text = scan.nextLine().trim();
                    strBuf.append(text);
                    strBuf = removeBlankSpace(strBuf);
                    final Node<Integer, String> nodeAppend = new Node<Integer, String>(command, text);
                    myStack.push(nodeAppend);
                    break;
                case 2:
                    final int textLen = scan.nextInt();
                    final Node<Integer, String> nodeErase = new Node<Integer, String>(command, strBuf.substring(strBuf.length() - textLen));
                    strBuf.delete(strBuf.length() - textLen, strBuf.length());
                    myStack.push(nodeErase);
                    break;
                case 3:
                    final int k = scan.nextInt();
                    System.out.println(strBuf.charAt(k - 1));
                    break;
                case 4:
                    final Node lastNode = myStack.pop();
                    // last being Append we Erase
                    if ((Integer) lastNode.action == 1)
                    {
                        final String lastText = (String) lastNode.text;
                        strBuf.delete(strBuf.length() - lastText.length(), strBuf.length());

                    }
                    else
                    // last being Erase we append
                    {
                        final String lastText = (String) lastNode.text;
                        strBuf.append(lastText);
                    }
                    break;
            }
        }
    }
}

Updated