Simple_Text_Editor.scala

/*
Simple Text Editor
-------------------------------
Link: https://www.hackerrank.com/contests/wissen-coding-challenge-2021/challenges/simple-text-editor
-------------------------------

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

append(W) - Append string W to the end of S.
delete(k) - Delete the last k characters of S.
print(k) - Print the kth character of S.
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
S = abcde
ops = ['1 fg','3 6','2 5', '4', '3 7', '4','3 4'] 

operation
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

The results should be printed as:

f
g
d

Input Format

The first line contains an integer, Q, denoting the number of operations.
Each line i of the Q subsequent lines (where 0 <= i <Q) defines an operation to be performed. Each operation starts with a single integer, t (where t in {1,2,3,4}), denoting a type of operation as defined in the Problem Statement above. If the operation requires an argument, t is followed by its space-separated argument. For example, if t=1 and W = 'abcd', line i will be 1 abcd.

Output Format

Each operation of type  must print the  character on a new line.

Sample Input

STDIN   Function
-----   --------
8       Q = 8
1 abc   ops[0] = '1 abc'
3 3     ops[1] = '3 3'
2 3     ...
1 xy
3 2
4 
4 
3 1

Sample Output

c
y
a

Explanation

Initially, S is empty. The following sequence of 8 operations are described below:

S = "". We append abc to S, so S="abc".
Print the 3rd character on a new line. Currently, the 3rd character is c.
Delete the last 3 characters in  S(abc), so S="".
Append xy to S, so S="xy".
Print the 2nd character on a new line. Currently, the 2nd character is y.
Undo the last update to S, making S empty again (i.e., S="").
Undo the next to last update to S (the deletion of the last 3 characters), making S="abc".
Print the 1st character on a new line. Currently, the 1st character is a.

*/
(Scala Solution)

import scala.io.StdIn.{readInt,readLine}

object Solution {
    def textEditor(x: List[String], q:Array[String]): List[String] = q(0) match {
        case "1" => (x.head++q(1)) :: x
        case "2" => x.head.dropRight(q(1).toInt) :: x
        case "3" => {println(x.head(q(1).toInt-1)); x}
        case _ => x.tail
    }
    def main(args: Array[String]) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution
*/ 
        var x = List("")
        for (_ <- 1 to readInt) {
            x = textEditor(x, readLine.split(" "))
        }
    }
}