# Generate an N-length string having longest palindromic substring of length K

• Difficulty Level : Medium
• Last Updated : 09 Sep, 2021

Given two integers N and K (K ≤ N), the task is to obtain a string of length N such that maximum length of a palindromic substring of this string is K.

Examples:

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Input: N = 5, K = 3
Output: “abacd”
Explanation: Palindromic substrings are “a”, “b”, “c”, “d” and “aba”. Therefore, the longest palindromic substring from the given string is of length 3.

Input: N = 8, K = 4
Output: “abbacdef”
Explanation: Palindromic substrings are “a”, “b”, “c”, “d”, “e”, “f”, “bb”, “abba”. Therefore, the longest palindromic substring from the given string is of length 4.

Approach: The idea is based on the following observation that the string of any length made up of a single character is always palindromic, e.g. {‘a’, ‘bbbbb’, ‘ccc’}. So, in order to generate a string with required conditions, print ‘a’ K times such that it has a longest palindromic substring of length K fill the remaining N – K slots by a non-palindromic sequence.

Follow the steps below to solve the problem:

• Print ‘a’ exactly K times.
• Consider a non-palindromic sequence, say “bcd”.
• Print the string.

Below is the implementation of the above approach:

## C++

 `// C++ program to implement the above approach` `#include ``using` `namespace` `std;` `// Function to generate a string of``// length N having longest palindromic``// substring of length K``void` `string_palindrome(``int` `N, ``int` `K)``{` `    ``// Fill first K characters with 'a'``    ``for` `(``int` `i = 0; i < K; i++)``        ``cout << ``"a"``;` `    ``// Stores a non-palindromic sequence``    ``// to be repeated for N - k slots``    ``string s = ``"bcd"``;` `    ``// Print N - k remaining characters``    ``for` `(``int` `i = 0; i < N - K; i++)``        ``cout << s[i % 3];``}` `// Driver Code``int` `main()``{` `    ``// Given N and K``    ``int` `N = 5, K = 3;``    ``string_palindrome(N, K);` `    ``return` `0;``}`

## Java

 `// Java program to implement the above approach``import` `java.util.*;` `class` `GFG``{` `// Function to generate a String of``// length N having longest palindromic``// subString of length K``static` `void` `String_palindrome(``int` `N, ``int` `K)``{` `    ``// Fill first K characters with 'a'``    ``for` `(``int` `i = ``0``; i < K; i++)``        ``System.out.print(``"a"``);` `    ``// Stores a non-palindromic sequence``    ``// to be repeated for N - k slots``    ``String s = ``"bcd"``;` `    ``// Print N - k remaining characters``    ``for` `(``int` `i = ``0``; i < N - K; i++)``        ``System.out.print(s.charAt(i % ``3``));``}` `// Driver Code``public` `static` `void` `main(String[] args)``{` `    ``// Given N and K``    ``int` `N = ``5``, K = ``3``;``    ``String_palindrome(N, K);``}``}` `// This code is contributed by 29AjayKumar`

## Python3

 `# Python3 program to implement the above approach` `# Function to generate a string of``# length N having longest palindromic``# substring of length K``def` `string_palindrome(N, K):` `    ``# Fill first K characters with 'a'``    ``for` `i ``in` `range``(K):``        ``print``(``"a"``, end ``=` `"")` `    ``# Stores a non-palindromic sequence``    ``# to be repeated for N - k slots``    ``s ``=` `"bcd"` `    ``# PrN - k remaining characters``    ``for` `i ``in` `range``(N ``-` `K):``        ``print``(s[i ``%` `3``], end ``=` `"")` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``  ` `    ``# Given N and K``    ``N, K ``=` `5``, ``3``    ``string_palindrome(N, K)` `    ``# This code is contributed by mohit kumar 29`

## C#

 `// C# program to implement the above approach``using` `System;``class` `GFG``{``    ` `    ``// Function to generate a String of``    ``// length N having longest palindromic``    ``// subString of length K``    ``static` `void` `String_palindrome(``int` `N, ``int` `K)``    ``{``    ` `        ``// Fill first K characters with 'a'``        ``for` `(``int` `i = 0; i < K; i++)``            ``Console.Write(``"a"``);``    ` `        ``// Stores a non-palindromic sequence``        ``// to be repeated for N - k slots``        ``string` `s = ``"bcd"``;``    ` `        ``// Print N - k remaining characters``        ``for` `(``int` `i = 0; i < N - K; i++)``            ``Console.Write(s[i % 3]);``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `Main(``string``[] args)``    ``{``    ` `        ``// Given N and K``        ``int` `N = 5, K = 3;``        ``String_palindrome(N, K);``    ``}``}` `// This code is contributed by AnkThon`

## Javascript

 ``
Output:
`aaabc`

Time complexity: O(N)
Auxiliary space: O(1)

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