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Nth Even length Palindrome
• Difficulty Level : Medium
• Last Updated : 29 Apr, 2021

Given a number n as a string, find the nth even-length positive palindrome number .

Examples:

```Input : n = "1"
Output : 11
1st even-length palindrome is 11 .

Input : n = "10"
Output : 1001
The first 10 even-length palindrome numbers are 11, 22,
33, 44, 55, 66, 77, 88, 99 and 1001.```

As, it is a even-length palindrome so its first half should be equal to reverse of second half and length will be 2, 4, 6, 8 …. To evaluate nth palindrome let’s just see 1st 10 even-length palindrome numbers 11, 22, 33, 44, 55, 66, 77, 88, 99 and 1001 . Here, nth palindrome is nn’ where n’ is reverse of n . Thus we just have to write n and n’ in a consecutive manner where n’ is reverse of n .

Below is the implementation of this approach.

## C++

 `// C++ program to find n=th even length string.``#include ``using` `namespace` `std;` `// Function to find nth even length Palindrome``string evenlength(string n)``{``    ``// string r to store resultant``    ``// palindrome. Initialize same as s``    ``string res = n;` `    ``// In this loop string r stores``    ``// reverse of string s after the``    ``// string s in consecutive manner .``    ``for` `(``int` `j = n.length() - 1; j >= 0; --j)``        ``res += n[j];` `    ``return` `res;``}` `// Driver code``int` `main()``{``    ``string n = ``"10"``;``  ` `    ``// Function call``    ``cout << evenlength(n);``    ``return` `0;``}`

## Java

 `// Java program to find nth even length Palindrome``import` `java.io.*;` `class` `GFG``{``    ``// Function to find nth even length Palindrome``    ``static` `String evenlength(String n)``    ``{``        ``// string r to store resultant``        ``// palindrome. Initialize same as s``        ``String res = n;` `        ``// In this loop string r stores``        ``// reverse of string s after the``        ``// string s in consecutive manner``        ``for` `(``int` `j = n.length() - ``1``; j >= ``0``; --j)``            ``res += n.charAt(j);` `        ``return` `res;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String n = ``"10"``;``      ` `        ``// Function call``        ``System.out.println(evenlength(n));``    ``}``}` `// Contributed by Pramod Kumar`

## Python3

 `# Python3 program to find n=th even``# length string.``import` `math as mt` `# Function to find nth even length``# Palindrome`  `def` `evenlength(n):` `    ``# string r to store resultant``    ``# palindrome. Initialize same as s``    ``res ``=` `n` `    ``# In this loop string r stores``    ``# reverse of string s after the``    ``# string s in consecutive manner .``    ``for` `j ``in` `range``(``len``(n) ``-` `1``, ``-``1``, ``-``1``):``        ``res ``+``=` `n[j]` `    ``return` `res`  `# Driver code``n ``=` `"10"` `# Function call``print``(evenlength(n))` `# This code is contributed by``# Mohit kumar 29`

## C#

 `// C# program to find nth even``// length Palindrome``using` `System;` `class` `GFG {` `    ``// Function to find nth even``    ``// length Palindrome``    ``static` `string` `evenlength(``string` `n)``    ``{` `        ``// string r to store resultant``        ``// palindrome. Initialize same``        ``// as s``        ``string` `res = n;` `        ``// In this loop string r stores``        ``// reverse of string s after``        ``// the string s in consecutive``        ``// manner``        ``for` `(``int` `j = n.Length - 1; j >= 0; --j)``            ``res += n[j];` `        ``return` `res;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``string` `n = ``"10"``;` `        ``// Function call``        ``Console.WriteLine(evenlength(n));``    ``}``}` `// This code is contributed by vt_m.`

## PHP

 `= 0; --``\$j``)``        ``\$res` `= ``\$res` `. ``\$n``[``\$j``];` `    ``return` `\$res``;``}` `// Driver code``\$n` `= ``"10"``;` `// Function call``echo` `evenlength(``\$n``);` `// This code is contributed by ita_c``?>`

## Javascript

 ``
Output

`1001`

Time Complexity: O(n)

This article is contributed by Surya Priy. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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