Simple Text Editor - HackerRank

https://www.hackerrank.com/challenges/simple-text-editor

In this problem, your task is to implement a simple text editor. Initially, a file contains an empty string S$S$. Your task is to perform Q$Q$ operations consisting of the following 4$4$ types:

1. append(W)$(W)$ - Appends the string W$W$ at the end of S$S$.
2. erase(k)$(k)$ - Erase the last k$k$ character of S$S$.
3. get(k)$(k)$ - Returns the kth$k^{th}$ character of S$S$.
4. undo()$()$ - Undo the last not previously undone operation of type 1$1$ or 2$2$, so it reverts S$S$ to the state before that operation.

For each operation of type 3$3$, print a single line with the returned character of that operation.

Sample Input

9
1 abc
3 3
2 3
1 xy
3 2
4 
4 
3 1

Sample Output

c
y
a

https://www.hackerrank.com/rest/contests/master/challenges/simple-text-editor/hackers/antrromet/download_solution

        Scanner sc = new Scanner(System.in);
        int Q = sc.nextInt();
        sc.nextLine();
        StringBuilder s = new StringBuilder();
        Stack<String> stack = new Stack<String>();
        for(int i=0;i<Q;i++){
            String line = sc.nextLine();
            String[] split = line.split(" ");
            int t = Integer.valueOf(split[0]);
            switch(t){
                case 1:
                    String w = split[1];
                    s.append(w);
                    stack.push(1+" "+w.length());
                    break;
                case 2:
                    int k = Integer.valueOf(split[1]);
                    stack.push(2+" "+s.substring(s.length() - k));
                    s.setLength(s.length() - k);
                    break;
                case 3:
                    k = Integer.valueOf(split[1]);
                    System.out.println(s.charAt(k-1));
                    break;
                case 4:
                    String undo = stack.pop();
                    String undoItems[] = undo.split(" ");
                    if(undoItems[0].equals("1")){
                        s.setLength(s.length() - Integer.valueOf(undoItems[1]));
                    } else {
                        s.append(undoItems[1]);
                    }
                    break;
            }
        }

https://www.hackerrank.com/rest/contests/master/challenges/simple-text-editor/hackers/akshaynayak/download_solution

        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        Stack<String> st=new Stack<String>();
        st.push("");
        int k=-1;
        String prev="";
        
        for(int i=0;i<n;i++){
            int op=sc.nextInt();
            switch(op){
                case 1:
                String newstr=sc.next();
                prev=st.peek();
                st.push(prev+newstr);
                break;
                
                case 2:
                k=sc.nextInt();
                prev=st.peek();
                String temp=prev.substring(0,prev.length()-k);
                st.push(temp);
                break;
                
                case 3:
                k=sc.nextInt();
                System.out.println(st.peek().charAt(k-1));
                break;
                
                case 4:
                st.pop();
                break;
            }

https://www.hackerrank.com/rest/contests/master/challenges/simple-text-editor/hackers/MarcGoVi/download_solution

        Scanner sc = new Scanner(System.in);
        int q = sc.nextInt();
        Stack<String> stack = new Stack<String>();
        String s = "";
        
        for (int x = 0; x < q; x++) {
            int ope = sc.nextInt();
            switch (ope) {
                case 1: //append
                    stack.push(s);
                    String append = sc.next();
                    s += append;
                    break;
                case 2: //erase last x characters
                    stack.push(s);
                    int character = sc.nextInt();
                    s = s.substring(0, s.length() - character);
                    break;
                case 3: //print
                    int index = sc.nextInt();
                    System.out.println(s.charAt(index - 1));
                    break;
                case 4: //undo
                    s = stack.pop();
                    break;
                default:
                    break;
            }
        }

http://www.martinkysel.com/hackerrank-simple-text-editor-solution/

I maintain two separate stacks. Self.stack represents the current state of the text. I can read data from this stack by indexing it. Self.cmds keeps the history of commands. Each command type knows how to execute itself and how to revert itself.

class AppendCmd():

    def execute(self, stack, value):

        self.length = len(value)

        stack.extend(value)

         

    def revert(self, stack):

        for _ in xrange(self.length):

            stack.pop()

 

class EraseCmd():

    def execute(self, stack, value):

        self.text = stack[-value:]

        for _ in xrange(value):

            stack.pop()

         

    def revert(self, stack):

        stack.extend(self.text)        

             

class CustomStack:

    def __init__(self):

        self.stack = []

        self.cmds = []

         

    def push(self, value):

        cmd = AppendCmd()

        cmd.execute(self.stack, value)

        self.cmds.append(cmd)

     

    def pop(self, value):

        cmd = EraseCmd()

        cmd.execute(self.stack, value)

        self.cmds.append(cmd)

         

    def get(self, value):

        print self.stack[value]

     

    def revert(self):

        cmd = self.cmds.pop()

        cmd.revert(self.stack)

 

def main():   

     

    cs = CustomStack()

     

    N = int(raw_input())

     

    for _ in xrange(N):

        unknown = raw_input()

         

        command = unknown[0]

         

        if command == '1':

            cmd, value = unknown.split()

            cs.push(list(value))

        elif command == '2':

            cmd, value = map(int, unknown.split())

            cs.pop(value)

        elif command == '3':

            cmd, value = map(int, unknown.split())

            cs.get(value - 1)

        else:

            cs.revert()