# Find a palindromic string B such that given String A is a subsequense of B

• Difficulty Level : Medium
• Last Updated : 28 Jul, 2021

Given a string . Find a string , where B is a palindrome and A is a subsequence of B.

A subsequence of a string is a string that can be derived from it by deleting some (not necessarily consecutive) characters without changing the order of the remaining characters. For example, “cotst” is a subsequence of “contest”.

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A palindrome is a string that reads the same forward or backward.

Examples

```Input : A = "aba"
Output : B = aba
Explanation : "aba" is a subsequence of "aba"
which is a palindrome.

Input : A = "ab"
Output : B = abba```

Approach: Let reverse(s) be the reverse of a string . Now, s + reverse(s) will always have as a subsequence (as first half) and it is a palindrome.
Therefore, B = A + reverse(A).

Below is the implementation of the above approach:

## C++14

 `// C++ program to find a palindromic string B``// such that given String A is a subsequense of B``#include ``using` `namespace` `std;` `// Function to check if a string is palindrome``bool` `checkPalindrome(string s)``{``    ``// Reversing a string``    ``string x = s;``    ``reverse(s.begin(), s.end());` `    ``// check if reversed string is equal``    ``// to given string``    ``return` `s == x;``}` `// Function to find a palindromic string B``// such that given String A is a subsequense of B``string findStringB(string A)``{``    ``// Reversing the string A``    ``string B = A;``    ``reverse(A.begin(), A.end());``    ``A = A + B;` `    ``// If the string A is already a palindrome``    ``// return A``    ``if` `(checkPalindrome(B))``        ``return` `B;` `    ``// else return B``    ``return` `A;``}` `string reverse(string input)``{``    ``string temparray = input;``    ``int` `left, right = 0;``    ``right = temparray.length() - 1;` `    ``for` `(left = 0; left < right; left++, right--)` `        ``// Swap values of left and right``        ``swap(temparray[left], temparray[right]);` `    ``return` `temparray;``}` `// Driver Code``int` `main(``int` `argc, ``char` `const` `*argv[])``{``    ``string A = ``"ab"``;``    ``cout << findStringB(A) << endl;``    ``return` `0;``}` `// This code is contributed by``// sanjeev2552`

## Java

 `// Java program to find a palindromic string B``// such that given String A is a subsequense of B` `class` `GFG``{` `    ``// Function to check if a string is palindrome``    ``static` `boolean` `checkPalindrome(String s)``    ``{``        ``// Reversing a string``        ``String x = reverse(s);``        ` `        ``// check if reversed string is equal``        ``// to given string``        ``return` `x.equals(s);``    ``}``    ` `    ``// Function to find a palindromic string B``    ``// such that given String A is a subsequense of B``    ``static` `String findStringB(String A)``    ``{` `        ``// Reversing the string A``        ``String B = reverse(A);``        ``B = B + A;` `        ``// If the string A is already a palindrome``        ``// return A``        ``if` `(checkPalindrome(A))``        ``{``            ``return` `A;``        ``}` `        ``// else return B``        ``return` `B;``    ``}` `    ``static` `String reverse(String input)``    ``{``        ``char``[] temparray = input.toCharArray();``        ``int` `left, right = ``0``;``        ``right = temparray.length - ``1``;` `        ``for` `(left = ``0``; left < right; left++, right--)``        ``{``            ``// Swap values of left and right``            ``char` `temp = temparray[left];``            ``temparray[left] = temparray[right];``            ``temparray[right] = temp;``        ``}``        ``return` `String.valueOf(temparray);``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String A = ``"ab"``;``        ``System.out.println(findStringB(A));``    ``}``}` `// This code contributed by Rajput-Ji`

## Python3

 `# Python program to find a palindromic string B``# such that given String A is a subsequense of B` `# Function to check if a string is palindrome``def` `checkPalindrome(s):``    ``# Reversing a string``    ``x ``=` `s[::``-``1``]``    ``# check if reversed string is equal``    ``# to given string``    ``if``(x ``=``=` `s):``        ``return` `True``    ``else``:``        ``return` `False``        ` `# Function to find a palindromic string B``# such that given String A is a subsequense of B``def` `findStringB(A):``    ` `    ``# Reversing the string A``    ``B ``=` `A[::``-``1``]``    ` `    ``B ``=` `B ``+` `A``    ` `    ``# If the string A is already a palindrome``    ``# return A``    ``if``(checkPalindrome(A)):``        ``return` `A``      ` `    ``# else return B   ``    ``return` `B` `# Driver Code``A ``=``"ab"``print``(findStringB(A))`

## C#

 `// C# program to find a palindromic string B``// such that given String A is a subsequense of B``using` `System;` `class` `GFG``{`` ` `    ``// Function to check if a string is palindrome``    ``static` `bool` `checkPalindrome(String s)``    ``{``        ``// Reversing a string``        ``String x = reverse(s);``         ` `        ``// check if reversed string is equal``        ``// to given string``        ``return` `x.Equals(s);``    ``}``     ` `    ``// Function to find a palindromic string B``    ``// such that given String A is a subsequense of B``    ``static` `String findStringB(String A)``    ``{`` ` `        ``// Reversing the string A``        ``String B = reverse(A);``        ``B = B + A;`` ` `        ``// If the string A is already a palindrome``        ``// return A``        ``if` `(checkPalindrome(A))``        ``{``            ``return` `A;``        ``}`` ` `        ``// else return B``        ``return` `B;``    ``}`` ` `    ``static` `String reverse(String input)``    ``{``        ``char``[] temparray = input.ToCharArray();``        ``int` `left, right = 0;``        ``right = temparray.Length - 1;`` ` `        ``for` `(left = 0; left < right; left++, right--)``        ``{``            ``// Swap values of left and right``            ``char` `temp = temparray[left];``            ``temparray[left] = temparray[right];``            ``temparray[right] = temp;``        ``}``        ``return` `String.Join(``""``,temparray);``    ``}``     ` `    ``// Driver Code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``String A = ``"ab"``;``        ``Console.WriteLine(findStringB(A));``    ``}``}` `// This code has been contributed by 29AjayKumar`

## Javascript

 ``
Output:
`baab`

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