Decrypt a string according to given rules

Given encrypted string str, the task is to decrypt the given string when the encryption rules are as follows:  

1. Start with the first character of the original string.
2. In every odd step, append the next character to it.
3. In every even step, prepend the next character to the encrypted string so far.

For example, if str = “geeks” then the encrypted string will be, 
g -> ge -> ege -> egek -> segek 

Examples:

Input: str = “segosegekfrek” 
Output: geeksforgeeks

Input: str = “vrstie” 
Output: strive

Approach: The steps that are given to encrypt the string can be followed in reverse order to obtain the original string. There will be two conditions to obtain the original string:  

• If the string is of odd length, in the odd step add the characters from the back in the resultant string, else add from the front.
• If the string is of even length, in the odd step add characters from the front and add characters from the back in the even step.

The reverse of the resultant string thus obtained is the original string which was encrypted. 

Below is the implementation of the above approach: 

C++

// C++ program to decrypt the original string
#include <bits/stdc++.h>
using namespace std;

// Function to return the original string
// after decryption
string decrypt(string s, int l)
{
    // Stores the decrypted string
    string ans = "";

    // If length is odd
    if (l % 2) {

        // Step counter
        int cnt = 0;

        // Starting and ending index
        int indl = 0, indr = l - 1;

        // Iterate till all characters
        // are decrypted
        while (ans.size() != l) {

            // Even step
            if (cnt % 2 == 0)
                ans += s[indl++];

            // Odd step
            else
                ans += s[indr--];

            cnt++;
        }
    }

    // If length is even
    else {

        // Step counter
        int cnt = 0;

        // Starting and ending index
        int indl = 0, indr = l - 1;
        while (ans.size() != l) {

            // Even step
            if (cnt % 2 == 0)
                ans += s[indr--];

            // Odd step
            else
                ans += s[indl++];

            cnt++;
        }
    }

    // Reverse the decrypted string
    reverse(ans.begin(), ans.end());

    return ans;
}

// Driver Code
int main()
{
    string s = "segosegekfrek";
    int l = s.length();
    cout << decrypt(s, l);

    return 0;
}

Java

// Java program to decrypt the original string
import java.io.*;
import java.util.*;

class GFG
{

  // Function to return the original string
  // after decryption
  public static String decrypt(String s, int l)
  {

    // Stores the decrypted string
    String ans = "";

    // If length is odd
    if (l % 2 == 1)
    {

      // Step counter
      int cnt = 0;

      // Starting and ending index
      int indl = 0, indr = l - 1;

      // Iterate till all characters
      // are decrypted
      while (ans.length() != l)
      {

        // Even step
        if (cnt % 2 == 0)
          ans += s.charAt(indl++);

        // Odd step
        else
          ans += s.charAt(indr--);
        cnt++;
      }
    }

    // If length is even
    else
    {

      // Step counter
      int cnt = 0;

      // Starting and ending index
      int indl = 0, indr = l - 1;
      while (ans.length() != l)
      {

        // Even step
        if (cnt % 2 == 0)
          ans += s.charAt(indr--);

        // Odd step
        else
          ans += s.charAt(indl++);

        cnt++;
      }
    }

    // Reverse the decrypted string
    StringBuffer sbr = new StringBuffer(ans);
    sbr.reverse();
    return sbr.toString();
  }

  // Driver code
  public static void main (String[] args)
  {
    String s = "segosegekfrek";
    int l = s.length();
    System.out.println(decrypt(s, l));
  }
}

// This Code is contributed by Manu Pathria

Python3

# Python3 program to decrypt
# the original string

# Function to return the
# original string after
# decryption
def decrypt(s, l):

    # Stores the decrypted
    # string
    ans = ""

    # If length is odd
    if (l % 2):

        # Step counter
        cnt = 0

        # Starting and ending
        # index
        indl = 0
        indr = l - 1

        # Iterate till all
        # characters are decrypted
        while (len(ans) != l):

            # Even step
            if (cnt % 2 == 0):
                ans += s[indl]
                indl += 1

            # Odd step
            else:
                ans += s[indr]
                indr -= 1

            cnt += 1

    # If length is even
    else:

        # Step counter
        cnt = 0

        # Starting and ending
        # index
        indl = 0
        indr = l - 1
        while (len(ans) != l):

            # Even step
            if (cnt % 2 == 0):
                ans += s[indr]
                indr -= 1

            # Odd step
            else:
                ans += s[indl]
                indl += 1

            cnt += 1

    # Reverse the decrypted
    # string
    string = list(ans)
    string.reverse()
    ans = ''.join(string)

    return ans

# Driver Code
if __name__ == "__main__":

    s = "segosegekfrek"
    l = len(s)
    print(decrypt(s, l))

# This code is contributed by Chitranayal

C#

// C# program to decrypt the original string
using System;
using System.Collections;
using System.Collections.Generic;

class GFG
{

  // Function to return the original string
  // after decryption
  static string decrypt(string s, int l)
  {

    // Stores the decrypted string
    string ans = "";

    // If length is odd
    if (l % 2 == 1)
    {

      // Step counter
      int cnt = 0;

      // Starting and ending index
      int indl = 0, indr = l - 1;

      // Iterate till all characters
      // are decrypted
      while (ans.Length != l)
      {

        // Even step
        if (cnt % 2 == 0)
          ans += s[indl++];

        // Odd step
        else
          ans += s[indr--];
        cnt++;
      }
    }

    // If length is even
    else
    {

      // Step counter
      int cnt = 0;

      // Starting and ending index
      int indl = 0, indr = l - 1;
      while (ans.Length != l)
      {

        // Even step
        if (cnt % 2 == 0)
          ans += s[indr--];

        // Odd step
        else
          ans += s[indl++];

        cnt++;
      }
    }

    // Reverse the decrypted string
    //Array.Reverse(ans);
    char[] sbr = ans.ToCharArray();
    Array.Reverse(sbr);

    return new string(sbr);
  }

  // Driver code
  public static void Main ()
  {
    string s = "segosegekfrek";
    int l = s.Length;
    Console.Write(decrypt(s, l));
  }
}

// This Code is contributed by Samim Hossain Mondal.

Javascript

<script>

// JavaScript program to decrypt the original string

    // Function to return the original string
    // after decryption
      function decrypt( s , l) {

        // Stores the decrypted string
        var ans = "";

        // If length is odd
        if (l % 2 == 1) {

            // Step counter
            var cnt = 0;

            // Starting and ending index
            var indl = 0, indr = l - 1;

            // Iterate till all characters
            // are decrypted
            while (ans.length != l) {

                // Even step
                if (cnt % 2 == 0)
                    ans += s.charAt(indl++);

                // Odd step
                else
                    ans += s.charAt(indr--);
                cnt++;
            }
        }

        // If length is even
        else {

            // Step counter
            var cnt = 0;

            // Starting and ending index
            var indl = 0, indr = l - 1;
            while (ans.length != l) {

                // Even step
                if (cnt % 2 == 0)
                    ans += s.charAt(indr--);

                // Odd step
                else
                    ans += s.charAt(indl++);

                cnt++;
            }
        }

        // Reverse the decrypted string

        return ans.split('').reverse().join('');
    }

    // Driver code

        var s = "segosegekfrek";
        var l = s.length;
        document.write(decrypt(s, l));

// This code is contributed by todaysgaurav

</script>

Output

geeksforgeeks

Complexity Analysis:

• Time Complexity: O(N), as we are using a loop to traverse N times. Where N is the length of the string.
• Auxiliary Space: O(N), as we are using extra space for the string. Where N is the length of the string.

Below is Python’s implementation.

C++

#include <bits/stdc++.h>
using namespace std;

// Function for encryption
string encrypted_string(string strg)
{
    string EncryptedString = { strg[0] };
    for (int t = 1; t < strg.length(); t++) {
        if (t % 2 == 1) {
            EncryptedString.push_back(
                strg[t]); // At odd position, append the
                          // next character
        }
        else {
            EncryptedString.insert(
                0, 1, strg[t]); // At Even position, prepend
                                // the next character
        }
    }
    // print the resultant string
    return EncryptedString;
}

// Function for decryption
string decrypted_string(string strg)
{
    // Check the divisibility of length of string by 2
    if (strg.length() % 2 != 0) {
        string DecryptedString = { strg[0] };
        for (int t = 1; t < strg.length(); t++) {
            DecryptedString.push_back(
                strg[strg.length() - 1
                     - t]); // First add the characters from
                            // the back
            DecryptedString.push_back(
                strg[t]); // from the front
        }
        reverse(
            DecryptedString.begin(),
            DecryptedString.end()); // Reverse the final
                                    // string to get output
        return DecryptedString;
    }
    else {
        string reverse_strg = strg;
        reverse(
            reverse_strg.begin(),
            reverse_strg
                .end()); // In case of even length, reverse
                         // the given string first
        string DecryptedString = { reverse_strg[0] };

        // Looping over the reversed string
        for (int t = 1; t < reverse_strg.length(); t++) {
            DecryptedString.push_back(
                reverse_strg[reverse_strg.length() - 1
                             - t]); // First add the
                                    // characters from the
                                    // back
            DecryptedString.push_back(
                reverse_strg[t]); // and then from the front
        }
        reverse(
            DecryptedString.begin(),
            DecryptedString.end()); // Reverse the final
                                    // string to get output
        return DecryptedString;
    }
}

// Testing the algorithm with Driver Code
int main()
{
    cout << encrypted_string("geeks") << endl;
    cout << encrypted_string("geeksforgeeks") << endl;
    cout << encrypted_string("strive") << endl;
    cout << encrypted_string("vikaschitturi") << endl;
    cout << encrypted_string("padmachitturi") << endl;
    cout << encrypted_string("hackerrank") << endl;
    cout << encrypted_string("hackerearth") << endl;
    cout << encrypted_string("codechef") << endl;
    cout << encrypted_string("IamVikas") << endl;
    cout << encrypted_string("HelloHowareyou") << endl;
    cout << encrypted_string("whatareyoudoing") << endl;

    cout << decrypted_string("segek") << endl;
    cout << decrypted_string("segosegekfrek") << endl;
    cout << decrypted_string("vrstie") << endl;
    cout << decrypted_string("iuthskviacitr") << endl;
    cout << decrypted_string("iuthadpamcitr") << endl;
    cout << decrypted_string("nrechakrak") << endl;
    cout << decrypted_string("hreechakrat") << endl;
    cout << decrypted_string(encrypted_string("IamVikas"))
         << endl;
    cout << decrypted_string(
        encrypted_string("HelloHowareyou"))
         << endl;

    return 0;
}

Java

import java.util.*;

public class GFG {
    // Function for encryption
    public static String encryptedString(String str) {
        StringBuilder encryptedString = new StringBuilder();
        encryptedString.append(str.charAt(0));

        for (int t = 1; t < str.length(); t++) {
            if (t % 2 == 1) {
                encryptedString.append(str.charAt(t));
            } else {
                encryptedString.insert(0, str.charAt(t));
            }
        }

        return encryptedString.toString();
    }

    // Function for decryption
    public static String decryptedString(String str) {
        if (str.length() % 2 != 0) {
            StringBuilder decryptedString = new StringBuilder();
            decryptedString.append(str.charAt(0));

            for (int t = 1; t < str.length(); t++) {
                decryptedString.append(str.charAt(str.length() - 1 - t));
                decryptedString.append(str.charAt(t));
            }

            return decryptedString.reverse().toString();
        } else {
            StringBuilder reverseStr = new StringBuilder(str).reverse();
            StringBuilder decryptedString = new StringBuilder();
            decryptedString.append(reverseStr.charAt(0));

            for (int t = 1; t < reverseStr.length(); t++) {
                decryptedString.append(reverseStr.charAt(reverseStr.length() - 1 - t));
                decryptedString.append(reverseStr.charAt(t));
            }

            return decryptedString.reverse().toString();
        }
    }

    // Testing the algorithm with Driver Code
    public static void main(String[] args) {
        System.out.println(encryptedString("geeks"));
        System.out.println(encryptedString("geeksforgeeks"));
        System.out.println(encryptedString("strive"));
        System.out.println(encryptedString("vikaschitturi"));
        System.out.println(encryptedString("padmachitturi"));
        System.out.println(encryptedString("hackerrank"));
        System.out.println(encryptedString("hackerearth"));
        System.out.println(encryptedString("codechef"));
        System.out.println(encryptedString("IamVikas"));
        System.out.println(encryptedString("HelloHowareyou"));
        System.out.println(encryptedString("whatareyoudoing"));

        System.out.println(decryptedString("segek"));
        System.out.println(decryptedString("segosegekfrek"));
        System.out.println(decryptedString("vrstie"));
        System.out.println(decryptedString("iuthskviacitr"));
        System.out.println(decryptedString("iuthadpamcitr"));
        System.out.println(decryptedString("nrechakrak"));
        System.out.println(decryptedString("hreechakrat"));
        System.out.println(decryptedString(encryptedString("IamVikas")));
        System.out.println(decryptedString(encryptedString("HelloHowareyou")));
    }
}

Python3

#Python 3 Code for encrypting a string according to given rules
#Function for encryption
def encrypted_string(strg):
    EncryptedString = [strg[0]]
    for t in range(1,len(strg)):
        if (t%2 == 1):
            EncryptedString.append(strg[t]) #At odd position, append the next character
        else:
            EncryptedString.insert(0,strg[t]) #At Even position, prepend the next character
    #print the resultant string
    return (''.join(EncryptedString))


#Python 3 Code for decrypting a string (Answer to the actual question)
def decrypted_string(strg):
    #CHeck the divisibility of length of string by 2
    if (len(strg)%2!=0):
        DecryptedString = [strg[0]]
        for t in range(1,len(strg)):
            DecryptedString.append(strg[-1*t]) #First add the characters from the back
            DecryptedString.append(strg[t]) #from the front
        DecryptedString = DecryptedString[:len(strg)][::-1] #Reverse the final string to get output
        return (''.join(DecryptedString))
    else:
        reverse_strg = strg[::-1] #In case of even length, reverse the given string first
        DecryptedString=[reverse_strg[0]]

        #Looping over the reversed string
        for t in range(1,len(reverse_strg)):
            DecryptedString.append(reverse_strg[-1*t]) #First add the characters from the back
            DecryptedString.append(reverse_strg[t]) #and then from the front
        DecryptedString = DecryptedString[:len(reverse_strg)][::-1]
        return (''.join(DecryptedString))


#Testing the algorithm with Driver Code
if __name__== '__main__':

    print (encrypted_string('geeks'))
    print (encrypted_string('geeksforgeeks'))
    print (encrypted_string('strive'))
    print (encrypted_string('vikaschitturi'))
    print (encrypted_string('padmachitturi'))
    print (encrypted_string('hackerrank'))
    print (encrypted_string('hackerearth'))
    print (encrypted_string('codechef'))
    print (encrypted_string('IamVikas'))
    print (encrypted_string('HelloHowareyou'))
    print (encrypted_string('whatareyoudoing'))

    print(decrypted_string('segek'))
    print(decrypted_string('segosegekfrek'))
    print(decrypted_string('vrstie'))
    print(decrypted_string('iuthskviacitr'))
    print(decrypted_string('iuthadpamcitr'))
    print(decrypted_string('nrechakrak'))
    print(decrypted_string('hreechakrat'))
    print(decrypted_string(encrypted_string('IamVikas')))
    print(decrypted_string(encrypted_string('HelloHowareyou')))
    print(decrypted_string(encrypted_string('whatareyoudoing')))

C#

using System;
using System.Text;

class Program
{
    // Function for encryption
    static string EncryptedString(string str)
    {
        StringBuilder encryptedString = new StringBuilder(str[0].ToString());

        for (int i = 1; i < str.Length; i++)
        {
            if (i % 2 == 1)
            {
                encryptedString.Append(str[i]); // At odd position, append the next character
            }
            else
            {
                encryptedString.Insert(0, str[i].ToString()); // At even position, prepend the next character
            }
        }

        return encryptedString.ToString();
    }

    // Function for decryption
    static string DecryptedString(string str)
    {
        if (str.Length % 2 != 0)
        {
            StringBuilder decryptedString = new StringBuilder(str[0].ToString());

            for (int i = 1; i < str.Length; i++)
            {
                decryptedString.Append(str[str.Length - 1 - i]); // First add the characters from the back
                decryptedString.Append(str[i]); // from the front
            }

            char[] charArray = decryptedString.ToString().ToCharArray();
            Array.Reverse(charArray);
            return new string(charArray);
        }
        else
        {
            string reverseStr = new string(str.ToCharArray());
            Array.Reverse(reverseStr.ToCharArray());

            StringBuilder decryptedString = new StringBuilder(reverseStr[0].ToString());

            for (int i = 1; i < reverseStr.Length; i++)
            {
                decryptedString.Append(reverseStr[reverseStr.Length - 1 - i]); // First add the characters from the back
                decryptedString.Append(reverseStr[i]); // and then from the front
            }

            char[] charArray = decryptedString.ToString().ToCharArray();
            Array.Reverse(charArray);
            return new string(charArray);
        }
    }

    static void Main()
    {
        Console.WriteLine(EncryptedString("geeks"));
        Console.WriteLine(EncryptedString("geeksforgeeks"));
        Console.WriteLine(EncryptedString("strive"));
        Console.WriteLine(EncryptedString("vikaschitturi"));
        Console.WriteLine(EncryptedString("padmachitturi"));
        Console.WriteLine(EncryptedString("hackerrank"));
        Console.WriteLine(EncryptedString("hackerearth"));
        Console.WriteLine(EncryptedString("codechef"));
        Console.WriteLine(EncryptedString("IamVikas"));
        Console.WriteLine(EncryptedString("HelloHowareyou"));
        Console.WriteLine(EncryptedString("whatareyoudoing"));

        Console.WriteLine(DecryptedString("segek"));
        Console.WriteLine(DecryptedString("segosegekfrek"));
        Console.WriteLine(DecryptedString("vrstie"));
        Console.WriteLine(DecryptedString("iuthskviacitr"));
        Console.WriteLine(DecryptedString("iuthadpamcitr"));
        Console.WriteLine(DecryptedString("nrechakrak"));
        Console.WriteLine(DecryptedString("hreechakrat"));
        Console.WriteLine(DecryptedString(EncryptedString("IamVikas")));
        Console.WriteLine(DecryptedString(EncryptedString("HelloHowareyou")));
    }
}

Javascript

// Function for encryption
function encryptedString(strg) {
    let encryptedString = strg[0];

    for (let t = 1; t < strg.length; t++) {
        if (t % 2 === 1) {
            encryptedString += strg[t];
        } else {
            encryptedString = strg[t] + encryptedString;
        }
    }

    return encryptedString;
}

// Function for decryption
function decryptedString(strg) {
    if (strg.length % 2 !== 0) {
        let decryptedString = strg[0];

        for (let t = 1; t < strg.length; t++) {
            decryptedString += strg[strg.length - 1 - t];
            decryptedString += strg[t];
        }

        return decryptedString.split('').reverse().join('');
    } else {
        let reverseStrg = strg.split('').reverse().join('');
        let decryptedString = reverseStrg[0];

        for (let t = 1; t < reverseStrg.length; t++) {
            decryptedString += reverseStrg[reverseStrg.length - 1 - t];
            decryptedString += reverseStrg[t];
        }

        return decryptedString.split('').reverse().join('');
    }
}

// Testing the algorithm with Driver Code
console.log(encryptedString("geeks"));
console.log(encryptedString("geeksforgeeks"));
console.log(encryptedString("strive"));
console.log(encryptedString("vikaschitturi"));
console.log(encryptedString("padmachitturi"));
console.log(encryptedString("hackerrank"));
console.log(encryptedString("hackerearth"));
console.log(encryptedString("codechef"));
console.log(encryptedString("IamVikas"));
console.log(encryptedString("HelloHowareyou"));
console.log(encryptedString("whatareyoudoing"));

console.log(decryptedString("segek"));
console.log(decryptedString("segosegekfrek"));
console.log(decryptedString("vrstie"));
console.log(decryptedString("iuthskviacitr"));
console.log(decryptedString("iuthadpamcitr"));
console.log(decryptedString("nrechakrak"));
console.log(decryptedString("hreechakrat"));
console.log(decryptedString(encryptedString("IamVikas")));
console.log(decryptedString(encryptedString("HelloHowareyou")));

Output

segek
segosegekfrek
vrstie
iuthskviacitr
iuthadpamcitr
nrechakrak
hreechakrat
ecdcoehf
aimIaVks
oeaoolHelHwryu
gidoeaawhtryuon
geeks
geeksforgeeks
strive
vikaschitturi
padmachitturi
hackerrank
hackerearth
IamVikas
HelloHowareyou
whatareyoudoing

Complexity Analysis:

• Time Complexity: O(N), as we are using a loop to traverse N times. Where N is the length of the string.
• Auxiliary Space: O(N), as we are using extra space. Where N is the length of the string.

Approach: Using 2 pointers

C++

#include <bits/stdc++.h>
using namespace std;

string decrypt(string encrypted_string)
{
  int i = 0;
  int n = encrypted_string.size();
  int j = n - 1;
  string decrypted_string = "";
  if (n % 2 == 1) {
    while (i < j) {
      decrypted_string
        = encrypted_string[i] + decrypted_string;
      decrypted_string
        = encrypted_string[j] + decrypted_string;
      i++;
      j--;
    }
    decrypted_string
      = encrypted_string[i] + decrypted_string;
  }
  else {
    while (i < j) {
      decrypted_string
        = encrypted_string[j] + decrypted_string;
      decrypted_string
        = encrypted_string[i] + decrypted_string;
      i++;
      j--;
    }
  }
  return decrypted_string;
}
int main()
{
  cout << (decrypt("segosegekfrek")) << endl;
  cout << (decrypt("vrstie")) << endl;
  cout << (decrypt("segek")) << endl;
}

// This code is contributed by garg28harsh.

Java

/*package whatever //do not write package name here */
import java.util.*;

public class Solution {
    static String decrypt(String encrypted_string)
    {
        int i = 0;
        int n = encrypted_string.length();
        int j = n - 1;
        String decrypted_string = "";
        if (n % 2 == 1) {
            while (i < j) {
                decrypted_string
                    = encrypted_string.charAt(i)
                      + decrypted_string;
                decrypted_string
                    = encrypted_string.charAt(j)
                      + decrypted_string;
                i++;
                j--;
            }
            decrypted_string = encrypted_string.charAt(i)
                               + decrypted_string;
        }
        else {
            while (i < j) {
                decrypted_string
                    = encrypted_string.charAt(j)
                      + decrypted_string;
                decrypted_string
                    = encrypted_string.charAt(i)
                      + decrypted_string;
                i++;
                j--;
            }
        }
        return decrypted_string;
    }
    public static void main(String[] args)
    {
        System.out.println(decrypt("segosegekfrek"));
        System.out.println(decrypt("vrstie"));
        System.out.println(decrypt("segek"));
    }
}
// This code is contributed by karandeep1234

Python3

def decrypt(encrypted_string):
    i = 0
    n = len(encrypted_string)
    j = n-1
    decrypted_string = ''
    if n % 2 == 1:
        while i < j:
            decrypted_string = encrypted_string[i] + decrypted_string
            decrypted_string = encrypted_string[j] + decrypted_string
            i += 1
            j -= 1
        decrypted_string = encrypted_string[i] + decrypted_string
    else:
        while i < j:
            decrypted_string = encrypted_string[j] + decrypted_string
            decrypted_string = encrypted_string[i] + decrypted_string
            i += 1
            j -= 1
    return decrypted_string


# Testing sample inputs
if __name__ == '__main__':
    print(decrypt('segosegekfrek')) # should print geeksforgeeks
    print(decrypt('vrstie')) # should print strive
    print(decrypt('segek')) # should print geeks

C#

using System;

public class GFG {
  static string decrypt(string encrypted_string)
  {
    int i = 0;
    int n = encrypted_string.Length;
    int j = n - 1;
    String decrypted_string = "";
    if (n % 2 == 1) {
      while (i < j) {
        decrypted_string = encrypted_string[i]
          + decrypted_string;
        decrypted_string = encrypted_string[j]
          + decrypted_string;
        i++;
        j--;
      }
      decrypted_string
        = encrypted_string[i] + decrypted_string;
    }
    else {
      while (i < j) {
        decrypted_string = encrypted_string[j]
          + decrypted_string;
        decrypted_string = encrypted_string[i]
          + decrypted_string;
        i++;
        j--;
      }
    }
    return decrypted_string;
  }

  public static void Main(string[] args)
  {
    Console.WriteLine(decrypt("segosegekfrek"));
    Console.WriteLine(decrypt("vrstie"));
    Console.WriteLine(decrypt("segek"));
  }
}

// This code is contributed by karandeep1234

Javascript

function decrypt(encrypted_string)
{
  let i = 0;
  let n = encrypted_string.length;
  let j = n - 1;
  let decrypted_string = "";
  if (n % 2 == 1) {
    while (i < j) {
      decrypted_string = encrypted_string[i] + decrypted_string;
      decrypted_string = encrypted_string[j] + decrypted_string;
      i++;
      j--;
    }
    decrypted_string = encrypted_string[i] + decrypted_string;
  }
  else {
    while (i < j) {
      decrypted_string = encrypted_string[j] + decrypted_string;
      decrypted_string = encrypted_string[i] + decrypted_string;
      i++;
      j--;
    }
  }
  return decrypted_string;
}

console.log(decrypt("segosegekfrek"));
console.log(decrypt("vrstie"));
console.log(decrypt("segek"));

// This code is contributed by Samim Hossain Mondal.

Output

geeksforgeeks
strive
geeks

Complexity Analysis:

• Time Complexity: O(N), as we are using a loop to traverse N times. Where N is the length of the string.
• Auxiliary Space: O(N), as we are using extra space. Where N is the length of the string.

Decrypt a string according to given rules:

Approach:

1. Define a function “decrypt” that takes the “ciphertext” string as input.
2. Iterate over all the possible 26 shifts.
3. For each shift, create an empty string “decrypted”.
4. Iterate over each character in the “ciphertext” string.
5. If the character is an alphabet, then shift it using the current shift value and add it to the “decrypted” string.
6. If the character is not an alphabet, then add it directly to the “decrypted” string.
7. Print the current shift value and the decrypted message for that shift.
8. Time complexity: O(n^2) where n is the length of the “ciphertext” string since we are iterating over all the possible 26 shifts for each character in the string.
9. Space complexity: O(n) since we are creating a new string “decrypted” for each shift.

C++

#include <iostream>
#include <string>

using namespace std;

void decrypt(string ciphertext) {
  for (int shift = 0; shift < 26; shift++) {
    string decrypted = "";
    for (char& c : ciphertext) {
      if (isalpha(c)) {
        if (isupper(c)) {
          decrypted += ((c - shift - 'A' + 26) % 26) + 'A';
        } else {
          decrypted += ((c - shift - 'a' + 26) % 26) + 'a';
        }
      } else {
        decrypted += c;
      }
    }
    cout << "Shift: " << shift << ", Decrypted message: " << decrypted << endl;
  }
}

int main() {
  string ciphertext = "Khoor, zruog!";
  decrypt(ciphertext);
  return 0;
}

Java

import java.util.*;

public class Main {

  public static void decrypt(String ciphertext) {
    for (int shift = 0; shift < 26; shift++) {
      String decrypted = "";
      for (char c : ciphertext.toCharArray()) {
        if (Character.isLetter(c)) {
          if (Character.isUpperCase(c)) {
            decrypted += (char) (((c - shift - 'A' + 26) % 26) + 'A');
          } else {
            decrypted += (char) (((c - shift - 'a' + 26) % 26) + 'a');
          }
        } else {
          decrypted += c;
        }
      }
      System.out.println("Shift: " + shift + ", Decrypted message: " + decrypted);
    }
  }

  public static void main(String[] args) {
    String ciphertext = "Khoor, zruog!";
    decrypt(ciphertext);
  }
}

Python3

def decrypt(ciphertext):
    for shift in range(26):
        decrypted = ""
        for char in ciphertext:
            if char.isalpha():
                if char.isupper():
                    decrypted += chr((ord(char) - shift - 65) % 26 + 65)
                else:
                    decrypted += chr((ord(char) - shift - 97) % 26 + 97)
            else:
                decrypted += char
        print(f"Shift: {shift}, Decrypted message: {decrypted}")

ciphertext = "Khoor, zruog!"
decrypt(ciphertext)

C#

using System;

class Program {
    static void Main(string[] args)
    {
        string ciphertext
            = "Khoor, zruog!"; // The encrypted message
        Decrypt(ciphertext); // Call the Decrypt function
                             // with the encrypted message
                             // as an argument
    }

    static void Decrypt(string ciphertext)
    {
        for (int shift = 0; shift < 26;
             shift++) // Loop through all possible shifts
        {
            string decrypted
                = ""; // Initialize an empty string to store
                      // the decrypted message
            foreach(
                char c in ciphertext) // Loop through each
                                      // character in the
                                      // encrypted message
            {
                if (char.IsLetter(
                        c)) // Check if the character is a
                            // letter
                {
                    if (char.IsUpper(
                            c)) // Check if the letter is
                                // uppercase
                    {
                        decrypted
                            += (char)(((c - shift - 'A')
                                       + 26)
                                          % 26
                                      + 'A'); // Decrypt
                                              // uppercase
                                              // letters
                                              // using the
                                              // shift value
                    }
                    else {
                        decrypted
                            += (char)(((c - shift - 'a')
                                       + 26)
                                          % 26
                                      + 'a'); // Decrypt
                                              // lowercase
                                              // letters
                                              // using the
                                              // shift value
                    }
                }
                else {
                    decrypted += c;
                }
            }
            Console.WriteLine(
                "Shift: " + shift + ", Decrypted message: "
                + decrypted); // Print the shift value and
                              // decrypted message for each
                              // possible shift value
        }
    }
}

Javascript

function decrypt(ciphertext) {
  for (let shift = 0; shift < 26; shift++) {
    let decrypted = "";
    for (let i = 0; i < ciphertext.length; i++) {
      let char = ciphertext.charAt(i);
      if (char.match(/[a-z]/i)) {
        if (char === char.toUpperCase()) {
          decrypted += String.fromCharCode((char.charCodeAt(0) - shift - 65 + 26) % 26 + 65);
        } else {
          decrypted += String.fromCharCode((char.charCodeAt(0) - shift - 97 + 26) % 26 + 97);
        }
      } else {
        decrypted += char;
      }
    }
    console.log(`Shift: ${shift}, Decrypted message: ${decrypted}`);
  }
}

let ciphertext = "Khoor, zruog!";
decrypt(ciphertext);

Output

...: 3, Decrypted message: Hello, world!
Shift: 4, Decrypted message: Gdkkn, vnqkc!
Shift: 5, Decrypted message: Fcjjm, umpjb!
Shift: 6, Decrypted message: Ebiil, tloia!
Shift: 7, Decrypted message: Dahhk, sknhz!
Shift: 8, Decrypted message: Czggj, rjmgy!
Shift: 9, Decrypted message: Byffi, qilfx!
Shift: 10, Decrypted message: Axeeh, phkew!
Shift: 11, Decrypted message: Zwddg, ogjdv!
Shift: 12, Decrypted message: Yvccf, nficu!
Shift: 13, Decrypted message: Xubbe, mehbt!
Shift: 14, Decrypted message: Wtaad, ldgas!
Shift: 15, Decrypted message: Vszzc, kcfzr!
Shift: 16, Decrypted message: Uryyb, jbeyq!
Shift: 17, Decrypted message: Tqxxa, iadxp!
Shift: 18, Decrypted message: Spwwz, hzcwo!
Shift: 19, Decrypted message: Rovvy, gybvn!
Shift: 20, Decrypted message: Qnuux, fxaum!
Shift: 21, Decrypted message: Pmttw, ewztl!
Shift: 22, Decrypted message: Olssv, dvysk!
Shift: 23, Decrypted message: Nkrru, cuxrj!
Shift: 24, Decrypted message: Mjqqt, btwqi!
Shift: 25, Decrypted message: Lipps, asvph!

The time complexity of the “decrypt” function in the given Python code is O(n * 26), where n is the length of the input “ciphertext” string. This is because the function iterates over all the 26 possible shift values and for each shift value, it iterates over each character in the “ciphertext” string.

The auxiliary space of the function is also O(n * 26), where n is the length of the input “ciphertext” string. This is because for each shift value, a new string “decrypted” is created and its length is equal to the length of the “ciphertext” string. Therefore, the total space required by the function is proportional to the product of the length of the “ciphertext” string and the number of shift values, which is 26 in this case.