Rewrite of SimpleEditor in Haskell (without using exceptions)
stack test
==================================================
THE PROBLEM
Problem Statement
In this challenge, you must implement a simple text editor. Initially, your editor contains an empty string, S. You must 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.
Input Format
The first line contains an integer, Q, denoting the number of operations.
Each line i of the Q subsequent lines (where 1 <= i <= Q) defines an operation to be performed. Each operation starts with a single integer, t (where t is a member of set {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.
Constraints
- 1 <= Q <= 1000000
- 1 <= k <= S length
- The sum of the lengths of all W in the input <= 1000000
- The sum of k over all delete operations <= 2000000
- All input characters are lowercase English letters.
- It is guaranteed that the sequence of operations given as input is possible to perform.
Output Format
Each operation of type 3 must print the kth character on a new line.
Sample Input
8
1 abc
3 3
2 3
1 xy
3 2
4
4
3 1
Sample Output
c
y
a