# Decode the string encoded with the given algorithm

• Difficulty Level : Basic
• Last Updated : 31 May, 2021

Given a decoded string str which was decoded with the following encoding algorithm:
Write down the middle character of the string then delete it and repeat the process until there are no characters left. For example, “abba” will be encoded as “bbaa”
Note that the middle character is the first character of the two middle characters when the length of the string is even.
Examples:

```Input: "ofrsgkeeeekgs"
Output: geeksforgeeks

Input: str = "bbaa"
Output: abba```

Approach: It can be observed that while decoding the string, the first letter of the encoded string becomes the median of the decoded string. So first, write the very first character of the encoded string and remove it from the encoded string then start adding the first character of the encoded string first to the left and then to the right of the decoded string and do this task repeatedly till the encoded string becomes empty.
For example:

```Encoded String          Decoded String
ofrsgkeeeekgs           o
frsgkeeeekgs            fo
rsgkeeeekgs             for
sgkeeeekgs              sfor
gkeeeekgs               sforg
keeeekgs                ksforg
eeeekgs                 ksforge
eeekgs                  eksforge
eekgs                   eksforgee
ekgs                    eeksforgee
kgs                     eeksforgeek
gs                      geeksgorgeek
s                       geeksforgeeks                     ```

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `// Function to decode and print``// the original string``void` `decodeStr(string str, ``int` `len)``{` `    ``// To store the decoded string``    ``char` `c[len] = ``""``;``    ``int` `med, pos = 1, k;` `    ``// Getting the mid element``    ``if` `(len % 2 == 1)``        ``med = len / 2;``    ``else``        ``med = len / 2 - 1;` `    ``// Storing the first element of the``    ``// string at the median position``    ``c[med] = str;` `    ``// If the length is even then store``    ``// the second element also``    ``if` `(len % 2 == 0)``        ``c[med + 1] = str;` `    ``// k represents the number of characters``    ``// that are already stored in the c[]``    ``if` `(len & 1)``        ``k = 1;``    ``else``        ``k = 2;` `    ``for` `(``int` `i = k; i < len; i += 2) {``        ``c[med - pos] = str[i];` `        ``// If string length is odd``        ``if` `(len % 2 == 1)``            ``c[med + pos] = str[i + 1];` `        ``// If it is even``        ``else``            ``c[med + pos + 1] = str[i + 1];``        ``pos++;``    ``}` `    ``// Print the decoded string``    ``for` `(``int` `i = 0; i < len; i++)``        ``cout << c[i];``}` `// Driver code``int` `main()``{``    ``string str = ``"ofrsgkeeeekgs"``;``    ``int` `len = str.length();` `    ``decodeStr(str, len);` `    ``return` `0;``}`

## Java

 `// Java implementation of the approach``class` `GFG{` `// Function to decode and print``// the original String``static` `void` `decodeStr(String str, ``int` `len)``{` `    ``// To store the decoded String``    ``char` `[]c = ``new` `char``[len];``    ``int` `med, pos = ``1``, k;` `    ``// Getting the mid element``    ``if` `(len % ``2` `== ``1``)``        ``med = len / ``2``;``    ``else``        ``med = len / ``2` `- ``1``;` `    ``// Storing the first element of the``    ``// String at the median position``    ``c[med] = str.charAt(``0``);` `    ``// If the length is even then store``    ``// the second element also``    ``if` `(len % ``2` `== ``0``)``        ``c[med + ``1``] = str.charAt(``1``);` `    ``// k represents the number of characters``    ``// that are already stored in the c[]``    ``if` `(len % ``2` `== ``1``)``        ``k = ``1``;``    ``else``        ``k = ``2``;` `    ``for``(``int` `i = k; i < len; i += ``2``)``    ``{``       ``c[med - pos] = str.charAt(i);``       ` `       ``// If String length is odd``       ``if` `(len % ``2` `== ``1``)``           ``c[med + pos] = str.charAt(i + ``1``);``           ` `       ``// If it is even``       ``else``           ``c[med + pos + ``1``] = str.charAt(i + ``1``);``       ``pos++;``    ``}` `    ``// Print the decoded String``    ``for` `(``int` `i = ``0``; i < len; i++)``        ``System.out.print(c[i]);``}` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``String str = ``"ofrsgkeeeekgs"``;``    ``int` `len = str.length();` `    ``decodeStr(str, len);``}``}` `// This code is contributed by 29AjayKumar`

## Python3

 `# Python3 implementation of the``# above approach` `# Function to decode and print``# the original string``def` `decodeStr(``str``, ``len``):` `    ``# To store the decoded string``    ``c ``=` `["" ``for` `i ``in` `range``(``len``)]``    ``pos ``=` `1` `    ``# Getting the mid element``    ``if``(``len` `%` `2` `=``=` `1``):``        ``med ``=` `int``(``len` `/` `2``)``    ``else``:``        ``med ``=` `int``(``len` `/` `2` `-` `1``)` `    ``# Storing the first element ``    ``# of the string at the``    ``# median position``    ``c[med] ``=` `str``[``0``]` `    ``# If the length is even``    ``# then store the second``    ``# element also``    ``if``(``len` `%` `2` `=``=` `0``):``        ``c[med ``+` `1``] ``=` `str``[``1``]` `    ``# k represents the number``    ``# of characters that are``    ``# already stored in the c[]``    ``if``(``len` `& ``1``):``        ``k ``=` `1``    ``else``:``        ``k ``=` `2` `    ``for` `i ``in` `range``(k, ``len``, ``2``):``        ``c[med ``-` `pos] ``=` `str``[i]` `        ``# If string length is odd``        ``if``(``len` `%` `2` `=``=` `1``):``            ``c[med ``+` `pos] ``=` `str``[i ``+` `1``]` `        ``# If it is even``        ``else``:``            ``c[med ``+` `pos ``+` `1``] ``=` `str``[i ``+` `1``]``        ``pos ``+``=` `1` `    ``# Print the decoded string``    ``print``(``*``c, sep ``=` `"")` `# Driver code``str` `=` `"ofrsgkeeeekgs"``len` `=` `len``(``str``)``decodeStr(``str``, ``len``)` `# This code is contributed by avanitrachhadiya2155`

## C#

 `// C# implementation of the approach``using` `System;` `class` `GFG{` `// Function to decode and print``// the original String``static` `void` `decodeStr(String str, ``int` `len)``{` `    ``// To store the decoded String``    ``char` `[]c = ``new` `char``[len];``    ` `    ``int` `med, pos = 1, k;` `    ``// Getting the mid element``    ``if` `(len % 2 == 1)``        ``med = len / 2;``    ``else``        ``med = len / 2 - 1;` `    ``// Storing the first element of the``    ``// String at the median position``    ``c[med] = str;` `    ``// If the length is even then store``    ``// the second element also``    ``if` `(len % 2 == 0)``        ``c[med + 1] = str;` `    ``// k represents the number of characters``    ``// that are already stored in the c[]``    ``if` `(len % 2 == 1)``        ``k = 1;``    ``else``        ``k = 2;` `    ``for``(``int` `i = k; i < len; i += 2)``    ``{``       ``c[med - pos] = str[i];``       ` `       ``// If String length is odd``       ``if` `(len % 2 == 1)``           ``c[med + pos] = str[i + 1];``       ` `       ``// If it is even``       ``else``           ``c[med + pos + 1] = str[i + 1];``       ``pos++;``    ``}` `    ``// Print the decoded String``    ``for``(``int` `i = 0; i < len; i++)``       ``Console.Write(c[i]);``}` `// Driver code``public` `static` `void` `Main(String[] args)``{``    ``String str = ``"ofrsgkeeeekgs"``;``    ``int` `len = str.Length;` `    ``decodeStr(str, len);``}``}` `// This code is contributed by sapnasingh4991`

## Javascript

 ``

Output:

`geeksforgeeks`

The Complexity: O(n)

My Personal Notes arrow_drop_up