Skip to content
Related Articles

Related Articles

Save Article
Improve Article
Save Article
Like Article

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:

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.

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 <bits/stdc++.h>
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




<script>
 
// JavaScript program for above approach
 
    // Function to generate a String of
    // length N having longest palindromic
    // subString of length K
    function String_palindrome(N, K)
    {
      
        // Fill first K characters with 'a'
        for (let i = 0; i < K; i++)
            document.write("a");
      
        // Stores a non-palindromic sequence
        // to be repeated for N - k slots
        let s = "bcd";
      
        // Print N - k remaining characters
        for (let i = 0; i < N - K; i++)
            document.write(s[i % 3]);
    }
 
 
// Driver Code
 
     // Given N and K
        let N = 5, K = 3;
        String_palindrome(N, K);
         
</script>
Output: 
aaabc

 

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




My Personal Notes arrow_drop_up
Recommended Articles
Page :