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

• Difficulty Level : Easy
• Last Updated : 17 Mar, 2023

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.

## Approach: Two-Pointer Swap

The steps to solve the problem using this approach are as follows:

1. Initialize two pointers i and j to 0 and 1, respectively. These pointers will help us to keep track of the current indices of the elements in the array that we are processing.
2. Loop through the array until i or j reaches the end of the array.
3. If the element at index i is odd and the element at index j is even, swap them. This will ensure that the even number is moved to an even index and odd number is moved to an odd index.
4. Increment both i and j by 2 to move them to the next odd and even index respectively.
5. If the element at index i is already even, increment i by 2.
6. Similarly, if the element at index j is already odd, increment j by 2.
7. Once we have looped through the entire array, the even numbers will be at even indices and odd numbers will be at odd indices.
8. Return the modified array.

## C++

 `#include ` `#include `   `using` `namespace` `std;`   `vector<``int``> arrange_array(vector<``int``> arr) {` `    ``int` `i = 0;` `    ``int` `j = 1;` `    ``int` `n = arr.size();` `    ``while` `(i < n && j < n) {` `        ``if` `(arr[i] % 2 != 0 && arr[j] % 2 == 0) {` `            ``swap(arr[i], arr[j]);` `            ``i += 2;` `            ``j += 2;` `        ``}` `        ``else` `{` `            ``if` `(arr[i] % 2 == 0) {` `                ``i += 2;` `            ``}` `            ``if` `(arr[j] % 2 != 0) {` `                ``j += 2;` `            ``}` `        ``}` `    ``}` `    ``return` `arr;` `}`   `int` `main() {` `    ``// Example 1` `    ``vector<``int``> arr1 = {3, 6, 12, 1, 5, 8};` `    ``vector<``int``> arranged_arr1 = arrange_array(arr1);` `    ``for` `(``auto` `i : arranged_arr1) {` `        ``cout << i << ``" "``;` `    ``}` `    ``cout << endl;`   `    ``// Example 2` `    ``vector<``int``> arr2 = {10, 9, 7, 18, 13, 19, 4, 20, 21, 14};` `    ``vector<``int``> arranged_arr2 = arrange_array(arr2);` `    ``for` `(``auto` `i : arranged_arr2) {` `        ``cout << i << ``" "``;` `    ``}` `    ``cout << endl;`   `    ``return` `0;` `}`

## Python3

 `def` `arrange_array(arr):` `    ``i ``=` `0` `    ``j ``=` `1` `    ``n ``=` `len``(arr)` `    ``while` `i < n ``and` `j < n:` `        ``if` `arr[i] ``%` `2` `!``=` `0` `and` `arr[j] ``%` `2` `=``=` `0``:` `            ``arr[i], arr[j] ``=` `arr[j], arr[i]` `            ``i ``+``=` `2` `            ``j ``+``=` `2` `        ``else``:` `            ``if` `arr[i] ``%` `2` `=``=` `0``:` `                ``i ``+``=` `2` `            ``if` `arr[j] ``%` `2` `!``=` `0``:` `                ``j ``+``=` `2` `    ``return` `arr`   `# Example 1` `arr ``=` `[``3``, ``6``, ``12``, ``1``, ``5``, ``8``]` `arranged_arr ``=` `arrange_array(arr)` `print``(arranged_arr)   ``# Output: [6, 3, 12, 1, 8, 5]`   `#Example` `arr ``=` `[``10``, ``9``, ``7``, ``18``, ``13``, ``19``, ``4``, ``20``, ``21``, ``14``]` `arranged_arr ``=` `arrange_array(arr)` `print``(arranged_arr)   ``# Output: [10, 9, 18, 7, 20, 19, 4, 13, 14, 21]`

Output

```[6, 3, 12, 1, 8, 5]
[10, 9, 18, 7, 20, 19, 4, 13, 14, 21]```

This algorithm runs in O(n) time complexity and O(1) space complexity

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