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

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: Please try your approach on {IDE} first, before moving on to the solution.

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 `

Output :

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

Time Complexity: O(n).
Auxiliary Space: O(1).

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

Don’t stop now and take your learning to the next level. Learn all the important concepts of Data Structures and Algorithms with the help of the most trusted course: DSA Self Paced. Become industry ready at a student-friendly price.

My Personal Notes arrow_drop_up

Improved By : 29AjayKumar

Article Tags :
Practice Tags :

4

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.