# Generate a string whose all K-size substrings can be concatenated to form the given string

Given a string str of size N and an integer K, the task is to generate a string whose substrings of size K can be concatenated to form the given string.

Examples:

Input: str = “abbaaa” K = 2
Output: abaa
Explanation:
All substring of size 2 of parent string “abaa” are “ab”, “ba” and “aa”. After concatenating all these substrings, the given string “abbaaa” can be obtained.

Input: str = “abcbcscsesesesd” K = 3
Output: abcsesd
Explanation :
All substring of size 3 of parent string “abcsesd” are “abc”, “bcs”, “cse”, “ses” and “esd”. After concatenating all these substrings, the given string “abcbcscsesesesd” can be obtained.

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach:
Follow the steps below to solve the problem:

1. We can clearly observe that by concatenating substrings of length K, except the first character, the remaining K-1 characters of any substring is present in the next substring as well.
2. Hence, traverse the string and append the first character of every substring to ans and then ignore next the K-1 characters.
3. Repeat this process for all substrings except the last substring.
4. Append all characters of the last substring to ans.
5. Return ans as the required decoded string.

Below is the implementation of the above approach :

## C++

 `// C++ program to generate a  ` `// string whose substrings of ` `// length K concatenates to  ` `// form given strings ` ` `  `#include   ` `using` `namespace` `std; ` ` `  `// Function to return the required  ` `// required string  ` `void` `decode_String(string str,  ` `int` `K) ` `{ ` `    ``string ans = ``""``; ` `    ``// Iterate the given string ` `    ``for` `(``int` `i = 0; i < str.size();  ` `    ``i += K) ` `        ``// Append the first  ` `        ``// character of every  ` `        ``// substring of length K ` `        ``ans += str[i]; ` ` `  `    ``// Consider all characters  ` `    ``// from the last substring ` `    ``for``(``int` `i = str.size() - (K - 1);  ` `    ``i < str.size(); i++) ` `        ``ans += str[i]; ` ` `  `    ``cout << ans << endl; ` `} ` ` `  `// Driver Program ` `int` `main() ` `{ ` `    ``int` `K = 3; ` `    ``string str = ``"abcbcscsesesesd"``; ` `    ``decode_String(str, K); ` `} `

## Java

 `// Java program to generate a  ` `// string whose substrings of  ` `// length K concatenates to  ` `// form given strings  ` `class` `GFG{ ` `     `  `// Function to return the required  ` `// required string  ` `public` `static` `void` `decode_String(String str, ` `                                 ``int` `K)  ` `{  ` `    ``String ans = ``""``;  ` `     `  `    ``// Iterate the given string  ` `    ``for``(``int` `i = ``0``;  ` `            ``i < str.length(); i += K)  ` `        `  `       ``// Append the first  ` `       ``// character of every  ` `       ``// substring of length K  ` `       ``ans += str.charAt(i);  ` `     `  `    ``// Consider all characters  ` `    ``// from the last substring  ` `    ``for``(``int` `i = str.length() - (K - ``1``); ` `            ``i < str.length(); i++)  ` `       ``ans += str.charAt(i);  ` `     `  `    ``System.out.println(ans); ` `}  ` ` `  `// Driver code ` `public` `static` `void` `main(String[] args) ` `{ ` `    ``int` `K = ``3``;  ` `    ``String str = ``"abcbcscsesesesd"``;   ` `     `  `    ``decode_String(str, K);  ` `} ` `} ` ` `  `// This code is contributed by divyeshrabadiya07 `

## C#

 `// C# program to generate a string ` `// whose substrings of length K ` `// concatenates to form given strings  ` `using` `System; ` ` `  `class` `GFG{ ` `     `  `// Function to return the required  ` `// required string  ` `public` `static` `void` `decode_String(String str, ` `                                 ``int` `K)  ` `{  ` `    ``String ans = ``""``;  ` `     `  `    ``// Iterate the given string  ` `    ``for``(``int` `i = 0;  ` `            ``i < str.Length; i += K)  ` `             `  `        ``// Append the first  ` `        ``// character of every  ` `        ``// substring of length K ` `        ``ans += str[i];  ` `     `  `    ``// Consider all characters  ` `    ``// from the last substring  ` `    ``for``(``int` `i = str.Length - (K - 1); ` `            ``i < str.Length; i++)  ` `        ``ans += str[i];  ` `     `  `    ``Console.WriteLine(ans); ` `}  ` ` `  `// Driver code ` `public` `static` `void` `Main(String[] args) ` `{ ` `    ``int` `K = 3;  ` `    ``String str = ``"abcbcscsesesesd"``;  ` `     `  `    ``decode_String(str, K);  ` `} ` `} ` ` `  `// This code is contributed by Rohit_ranjan `

Output:

```abcsesd
```

Time Complexity: O(N)
Auxiliary Space: O(1)

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