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.

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 <bits/stdc++.h>
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

Python3

# Python3 program to generate a
# string whose substrings of
# length K concatenates to
# form given strings

# Function to return the required
# required string
def decode_String(st, K):

    ans = ""

    # Iterate the given string
    for i in range(0, len(st), K):

        # Append the first
        # character of every
        # substring of length K
        ans += st[i]

    # Consider all characters
    # from the last substring
    for i in range(len(st) - (K - 1), len(st)):
        ans += st[i]

    print(ans)

# Driver code
if __name__ == "__main__":

    K = 3
    st = "abcbcscsesesesd"

    decode_String(st, K)

# This code is contributed by chitranayal

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

Javascript

<script>

// JavaScript program to implement
// the above approach

// Function to return the required
// required string
function decode_String(str, K)
{
    let ans = "";

    // Iterate the given string
    for(let 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(let i = str.length - (K - 1);
            i < str.length; i++)
        ans += str[i];

    document.write(ans);
}

// Driver code
    let K = 3;
    let str = "abcbcscsesesesd";

    decode_String(str, K);

 // This code is contributed by sanjoy_62.
</script>

Output:

abcsesd

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