# Even numbers at even index and odd numbers at odd index

• Difficulty Level : Easy
• Last Updated : 31 May, 2022

Given an array of size n containing equal number of odd and even numbers. The problem is to arrange the numbers in such a way that all the even numbers get the even index and odd numbers get the odd index. Required auxiliary space is O(1).
Examples :

```Input : arr[] = {3, 6, 12, 1, 5, 8}
Output : 6 3 12 1 8 5

Input : arr[] = {10, 9, 7, 18, 13, 19, 4, 20, 21, 14}
Output : 10 9 18 7 20 19 4 13 14 21 ```
Recommended Practice

Approach :

• Start from the left and keep two index one for even position and other for odd positions.
• Traverse these index from left.
• At even position there should be even number and at odd positions, there should be odd number.
• Whenever there is mismatch , we swap the values at odd and even index.

Below is the implementation of the above approach :

## CPP

 `// C++ implementation to arrange``// odd and even numbers``#include ``using` `namespace` `std;` `// function to arrange odd and even numbers``void` `arrangeOddAndEven(``int` `arr[], ``int` `n)``{``   ``int` `oddInd = 1;``    ``int` `evenInd = 0;``    ``while` `(``true``)``    ``{``        ``while` `(evenInd < n && arr[evenInd] % 2 == 0)``            ``evenInd += 2;``            ` `        ``while` `(oddInd < n && arr[oddInd] % 2 == 1)``            ``oddInd += 2;``            ` `        ``if` `(evenInd < n && oddInd < n)``            ``swap (arr[evenInd], arr[oddInd]);``            ` `        ``else``            ``break``;``    ``}``}` `// function to print the array``void` `printArray(``int` `arr[], ``int` `n)``{``    ``for` `(``int` `i = 0; i < n; i++)``        ``cout << arr[i] << ``" "``;``}` `// Driver program to test above``int` `main()``{``    ``int` `arr[] = { 3, 6, 12, 1, 5, 8 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``cout << ``"Original Array: "``;``    ``printArray(arr, n);` `    ``arrangeOddAndEven(arr, n);` `    ``cout << ``"\nModified Array: "``;``    ``printArray(arr, n);` `    ``return` `0;``}`

## Java

 `// Java implementation to``// arrange odd and even numbers` `import` `java.util.*;``import` `java.lang.*;` `class` `GfG {` `// function to arrange``// odd and even numbers``public` `static` `void` `arrangeOddAndEven(``int` `arr[], ``int` `n)``{``    ``int` `oddInd = ``1``;``    ``int` `evenInd = ``0``;``    ``while` `(``true``)``    ``{``        ``while` `(evenInd < n && arr[evenInd] % ``2` `== ``0``)``            ``evenInd += ``2``;``            ` `        ``while` `(oddInd < n && arr[oddInd] % ``2` `== ``1``)``            ``oddInd += ``2``;``            ` `        ``if` `(evenInd < n && oddInd < n)``            ``{``                ``int` `temp = arr[evenInd];``                ``arr[evenInd] = arr[oddInd];``                ``arr[oddInd] = temp;``            ``}``            ` `        ``else``            ``break``;``    ``}``}` `// function to print the array``public` `static` `void` `printArray(``int` `arr[], ``int` `n)``{``    ``for` `(``int` `i = ``0``; i < n; i++)``        ``System.out.print(arr[i] + ``" "``);``}``    ` `// Driver function``public` `static` `void` `main(String argc[]){``    ``int` `arr[] = { ``3``, ``6``, ``12``, ``1``, ``5``, ``8` `};``    ``int` `n = ``6``;` `    ``System.out.print(``"Original Array: "``);``    ``printArray(arr, n);` `    ``arrangeOddAndEven(arr, n);` `    ``System.out.print(``"\nModified Array: "``);``    ``printArray(arr, n);``}``}` `// This code is contributed by Sagar Shukla`

## Python3

 `    ` `# Python3 implementation to``# arrange odd and even numbers` `def` `arrangeOddAndEven(arr,  n):``    ` `    ``oddInd ``=` `1``    ``evenInd ``=` `0``    ``while` `(``True``):``        ` `        ``while` `(evenInd < n ``and` `arr[evenInd] ``%` `2` `=``=` `0``):``            ``evenInd ``+``=` `2``             ` `        ``while` `(oddInd < n ``and` `arr[oddInd] ``%` `2` `=``=` `1``):``            ``oddInd ``+``=` `2``             ` `        ``if` `(evenInd < n ``and` `oddInd < n):``                ``temp ``=` `arr[evenInd]``                ``arr[evenInd] ``=` `arr[oddInd]``                ``arr[oddInd] ``=` `temp;``             ` `        ``else``:``            ``break`` ` `# function to print the array``def` `printArray(arr,  n):``    ``for` `i ``in` `range``(``0``,n):``        ``print``(arr[i] , "``",end="``")` `     ` `# Driver function``def` `main():``    ``arr ``=` `[ ``3``, ``6``, ``12``, ``1``, ``5``, ``8` `]``    ``n ``=` `6`` ` `    ``print``(``"Original Array: "``,end``=``"")``    ``printArray(arr, n)`` ` `    ``arrangeOddAndEven(arr, n)`` ` `    ``print``(``"\nModified Array: "``,end``=``"")``    ``printArray(arr, n)``    ` `if` `__name__ ``=``=` `'__main__'``:``    ``main()``# This code is contributed by 29AjayKumar`

## C#

 `// C# implementation to``// arrange odd and even numbers``using` `System;` `class` `GFG {` `    ``// function to arrange``    ``// odd and even numbers``    ``public` `static` `void` `arrangeOddAndEven(``int``[] arr, ``int` `n)``    ``{``     ``int` `oddInd = 1;``     ``int` `evenInd = 0;``    ``while` `(``true``)``    ``{``        ``while` `(evenInd < n && arr[evenInd] % 2 == 0)``            ``evenInd += 2;``             ` `        ``while` `(oddInd < n && arr[oddInd] % 2 == 1)``            ``oddInd += 2;``             ` `        ``if` `(evenInd < n && oddInd < n)``            ``{``                ``int` `temp = arr[evenInd];``                ``arr[evenInd] = arr[oddInd];``                ``arr[oddInd] = temp;``            ``}``             ` `        ``else``            ``break``;``    ``}``    ``}` `    ``// function to print the array``    ``public` `static` `void` `printArray(``int``[] arr, ``int` `n)``    ``{``        ``for` `(``int` `i = 0; i < n; i++)``            ``Console.Write(arr[i] + ``" "``);``    ``}` `    ``// Driver function``    ``public` `static` `void` `Main()``    ``{``        ``int``[] arr = { 3, 6, 12, 1, 5, 8 };``        ``int` `n = 6;` `        ``Console.Write(``"Original Array: "``);``        ``printArray(arr, n);` `        ``arrangeOddAndEven(arr, n);` `        ``Console.Write(``"\nModified Array: "``);``        ``printArray(arr, n);``    ``}``}` `// This code is contributed by Sam007`

## Javascript

 ``

Output :

```Original Array: 3 6 12 1 5 8
Modified Array: 6 3 12 1 8 5 ```

Time Complexity: O(N), as we are using a loop to traverse N times.

Auxiliary Space: O(1), as we are not using any extra space.

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