Simple Text Editor HackerRank Solution in Java with Explanation

Problem Description :

Implement a simple text editor. The editor initially contains an empty string S. Perform Q operations of the following 4 types:

1. append (W) - Append string W to the end of S.
2. delete (k) - Delete the last K characters of S.
3. print (k) - Print the kth character of S.
4. undo () - Undo the last (not previously undone) operation of type 1 or 2, reverting S to the state it was in prior to that operation.

Example :

Input :

S = "abcde"
Operations = ["1 fg", "3 6", "2 5", "4", "3 7", "4", "3 4"]

index   S          ops[index]    explanation
-----   ------     ----------    -----------
0       abcde      1 fg          append fg
1       abcdefg    3 6           print the 6th letter - f
2       abcdefg    2 5           delete the last 5 letters
3       ab         4             undo the last operation, index 2
4       abcdefg    3 7           print the 7th characgter - g
5       abcdefg    4             undo the last operation, index 0
6       abcde      3 4           print the 4th character - d

Output :

f
g
d

See full problem description and on HackerRank site :

• Simple Text Editor
  

In this problem, we have to perform 4 operations on given String :

1. Add or Append
2. Delete the last characters from given number
3. Print the character of given index number
4. Undo last operation.

First 3 operations are easy, for 4th operation we have to maintain Stack. So we can undo last operation and store into given String S. So lets go...

Solution 1 : Simple Text Editor Solution in Java

import java.io.*;
import java.util.*;

public class Solution {

    public static void main(String[] args) throws NumberFormatException, IOException {
         BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
         Stack<String> stack = new Stack<>();
         int totalOp = Integer.parseInt(bufferedReader.readLine().trim());
         StringBuilder builder = new StringBuilder();
         
         while (totalOp-- > 0) {
            
             String s = bufferedReader.readLine().trim();
             int operation = Integer.parseInt(s.substring(0, 1));
             
             switch (operation) {
                case 1: {
                    String value = s.substring(2, s.length());
                    stack.push(builder.toString());
                    builder.append(value);
                    break;
                }
                case 2: {
                    stack.push(builder.toString());
                    String value = s.substring(2, s.length());
                    int index = Integer.parseInt(value);
                    builder.delete(builder.length()-index, builder.length());
                    break;
                }
                case 3: {
                    String value = s.substring(2, s.length());
                    int index = Integer.parseInt(value);
                    System.out.println(builder.charAt(index-1));
                    break;
                }
                case 4: {
                    StringBuilder b = new StringBuilder(stack.pop());
                    builder = b;
                    break;
                }
            }
        }
        bufferedReader.close();
    }
}

Solution Explanation :

• We maintains Stack for undo operations. So push data before append or delete characters from  String.
  
• First read input data from using BefferedReader.
• We have to perform operations till given number. so loop until totalOp becomes 0 in while loop.
  
• Get data one by one using readLine() method. (First column contains operation number).
• Now perform switch case on operations number.
1. In 1st operation, we have to perform append operation on given string.
   
   • Push current string data to Stack and append string to already given string.
2. Push current string data to Stack and delete characters from given number to string length.
   
3. Print given index character
   
4. Pop last string from Stack and assign to String. 

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