An improved version is below: 1. StringBuilder is used instead of creating many immutable strings (provides both space and performance improvements) 2. Instead of storing whole string in the stack , only the last update is stored: for additions: +(appended string length), for deletions: (deleted string). This drastically reduces total space used (although adding some extra processing).

import java.io.; import java.util.; public class Solution {

public static void main(String[] args) throws IOException{
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int n = Integer.parseInt(br.readLine());
StringBuilder sb = new StringBuilder();
Stack<String> stack = new Stack<>();
List<Character> result = new ArrayList<>();
for (int i = 0; i < n; i++) {
    String append = null;
    int show = 0;
    int del = 0;
    String inp = br.readLine();
    String arr[] = inp.split(" ");
    int opt = Integer.parseInt(arr[0]);
    switch (opt) {
    case 1:
        append = arr[1];
        sb.append(append);
        stack.push("+" + append.length()); // '+' is used to differentiate between append or delete operation
        break;
    case 2:
        del = Integer.parseInt(arr[1]);
        String delStr = sb.substring(sb.length() - del, sb.length());
        sb.delete(sb.length() - del, sb.length());
        stack.push(delStr);
        break;
    case 3:
        show = Integer.parseInt(arr[1]);
        result.add(sb.charAt(show - 1));
        break;
    case 4:
        String lastOp = stack.pop();
        if(lastOp.charAt(0) == '+') { // undo addition
            sb.delete(sb.length() - Integer.parseInt(lastOp.substring(1)), sb.length()); // omit first character, since it is just append operation marker (+)
        }
       else { // undo deletion
           sb.append(lastOp);
       }
    }
}
try {
    for(Character j:result)
        System.out.println(j);
} catch (Exception e) {

}
}

}