# Rearrange array such that even index elements are smaller and odd index elements are greater

• Difficulty Level : Easy
• Last Updated : 27 Jun, 2022

Given an array, rearrange the array such that :

1. If index i is even, arr[i] <= arr[i+1]
2. If index i is odd, arr[i] >= arr[i+1]

Note: There can be multiple answers.
Examples:

```Input  : arr[] = {2, 3, 4, 5}
Output : arr[] = {2, 4, 3, 5}
Explanation : Elements at even indexes are
smaller and elements at odd indexes are greater
than their next elements

is arr[] = {3, 4, 2, 5}

Input  :arr[] = {6, 4, 2, 1, 8, 3}
Output :arr[] = {4, 6, 1, 8, 2, 3}```

This problem is similar to sorting an array in the waveform.

• A simple solution is to sort the array in decreasing order, then starting from the second element, swap the adjacent elements.
• An efficient solution is to iterate over the array and swap the elements as per the given condition.

If we have an array of length n, then we iterate from index 0 to n-2 and check the given condition.
At any point of time if i is even and arr[i] > arr[i+1], then we swap arr[i] and arr[i+1]. Similarly, if i is odd and
arr[i] < arr[i+1], then we swap arr[i] and arr[i+1].
For the given example:

```Before rearrange, arr[] = {2, 3, 4, 5}
Start iterating over the array till index 2 (as n = 4) ```

First Step:
At i = 0, arr[i] = 2 and arr[i+1] = 3. As i is even and arr[i] < arr[i+1], don’t need to swap.
Second step:
At i = 1, arr[i] = 3 and arr[i+1] = 4. As i is odd and arr[i] < arr[i+1], swap them.
Now arr[] = {2, 4, 3, 5}
Third step:
At i = 2, arr[i] = 3 and arr[i+1] = 5. So, don’t need to swap them

After rearrange, arr[] = {2, 4, 3, 5}

Flowchart Flowchart

## C++

 `// CPP code to rearrange an array such that` `// even index elements are smaller and odd` `// index elements are greater than their` `// next.` `#include ` `using` `namespace` `std;`   `void` `rearrange(``int``* arr, ``int` `n)` `{` `    ``for` `(``int` `i = 0; i < n - 1; i++) {` `        ``if` `(i % 2 == 0 && arr[i] > arr[i + 1])` `            ``swap(arr[i], arr[i + 1]);`   `        ``if` `(i % 2 != 0 && arr[i] < arr[i + 1])` `            ``swap(arr[i], arr[i + 1]);` `    ``}` `}`   `/* Utility that prints out an array in` `   ``a line */` `void` `printArray(``int` `arr[], ``int` `size)` `{` `    ``for` `(``int` `i = 0; i < size; i++)` `        ``cout << arr[i] << ``" "``;`   `    ``cout << endl;` `}`   `/* Driver function to test above functions */` `int` `main()` `{` `    ``int` `arr[] = { 6, 4, 2, 1, 8, 3 };` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);`   `    ``cout << ``"Before rearranging: \n"``;` `    ``printArray(arr, n);`   `    ``rearrange(arr, n);`   `    ``cout << ``"After rearranging: \n"``;` `    ``printArray(arr, n);`   `    ``return` `0;` `}`

## Java

 `// Java code to rearrange an array such` `// that even index elements are smaller` `// and odd index elements are greater` `// than their next.`   `class` `GFG {`   `    ``static` `void` `rearrange(``int` `arr[], ``int` `n)` `    ``{`   `        ``int` `temp;` `        ``for` `(``int` `i = ``0``; i < n - ``1``; i++) {` `            ``if` `(i % ``2` `== ``0` `&& arr[i] > arr[i + ``1``]) {` `                ``temp = arr[i];` `                ``arr[i] = arr[i + ``1``];` `                ``arr[i + ``1``] = temp;` `            ``}` `            ``if` `(i % ``2` `!= ``0` `&& arr[i] < arr[i + ``1``]) {` `                ``temp = arr[i];` `                ``arr[i] = arr[i + ``1``];` `                ``arr[i + ``1``] = temp;` `            ``}` `        ``}` `    ``}`   `    ``/* Utility that prints out an array in` `    ``a line */` `    ``static` `void` `printArray(``int` `arr[], ``int` `size)` `    ``{` `        ``for` `(``int` `i = ``0``; i < size; i++)` `            ``System.out.print(arr[i] + ``" "``);`   `        ``System.out.println();` `    ``}`   `    ``// Driver code` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``int` `arr[] = { ``6``, ``4``, ``2``, ``1``, ``8``, ``3` `};` `        ``int` `n = arr.length;`   `        ``System.out.print(``"Before rearranging: \n"``);` `        ``printArray(arr, n);`   `        ``rearrange(arr, n);`   `        ``System.out.print(``"After rearranging: \n"``);` `        ``printArray(arr, n);` `    ``}` `}`   `// This code is contributed by Anant Agarwal.`

## Python3

 `# Python code to rearrange` `# an array such that` `# even index elements` `# are smaller and odd ` `# index elements are` `# greater than their ` `# next.`   `def` `rearrange(arr, n):`   `    ``for` `i ``in` `range``(n ``-` `1``):` `        ``if` `(i ``%` `2` `=``=` `0` `and` `arr[i] > arr[i ``+` `1``]):` `        `  `            ``temp ``=` `arr[i]` `            ``arr[i]``=` `arr[i ``+` `1``]` `            ``arr[i ``+` `1``]``=` `temp` `        `  `        ``if` `(i ``%` `2` `!``=` `0` `and` `arr[i] < arr[i ``+` `1``]):` `         `  `            ``temp ``=` `arr[i]` `            ``arr[i]``=` `arr[i ``+` `1``]` `            ``arr[i ``+` `1``]``=` `temp` `           `  ` `  `# Utility that prints out an array in` `# a line ` `def` `printArray(arr, size):`   `    ``for` `i ``in` `range``(size):` `        ``print``(arr[i], ``" "``, end ``=``"")` ` `  `    ``print``()`   `# Driver code`   `arr ``=` `[ ``6``, ``4``, ``2``, ``1``, ``8``, ``3` `]` `n ``=` `len``(arr)` ` `  `print``(``"Before rearranging: "``)` `printArray(arr, n)` ` `  `rearrange(arr, n)` ` `  `print``(``"After rearranging:"``)` `printArray(arr, n);`   `# This code is contributed` `# by Anant Agarwal.`

## C#

 `// C# code to rearrange an array such` `// that even index elements are smaller` `// and odd index elements are greater` `// than their next.` `using` `System;`   `class` `GFG {`   `    ``static` `void` `rearrange(``int``[] arr, ``int` `n)` `    ``{` `        ``int` `temp;` `        ``for` `(``int` `i = 0; i < n - 1; i++) {` `            ``if` `(i % 2 == 0 && arr[i] > arr[i + 1])` `            ``{` `                ``temp = arr[i];` `                ``arr[i] = arr[i + 1];` `                ``arr[i + 1] = temp;` `            ``}` `            `  `            ``if` `(i % 2 != 0 && arr[i] < arr[i + 1])` `            ``{` `                ``temp = arr[i];` `                ``arr[i] = arr[i + 1];` `                ``arr[i + 1] = temp;` `            ``}` `        ``}` `    ``}`   `    ``/* Utility that prints out an array in` `    ``a line */` `    ``static` `void` `printArray(``int``[] arr, ``int` `size)` `    ``{` `        ``for` `(``int` `i = 0; i < size; i++)` `            ``Console.Write(arr[i] + ``" "``);`   `        ``Console.WriteLine();` `    ``}`   `    ``// Driver code` `    ``public` `static` `void` `Main()` `    ``{` `        ``int``[] arr = { 6, 4, 2, 1, 8, 3 };` `        ``int` `n = arr.Length;`   `        ``Console.WriteLine(``"Before rearranging: "``);` `        ``printArray(arr, n);`   `        ``rearrange(arr, n);`   `        ``Console.WriteLine(``"After rearranging: "``);` `        ``printArray(arr, n);` `    ``}` `}`   `// This code is contributed by vt_m.`

## PHP

 ` ``\$arr``[``\$i` `+ 1])` `            ``swap(``\$arr``[``\$i``], ``\$arr``[``\$i` `+ 1]);`   `        ``if` `(``\$i` `% 2 != 0 && ` `            ``\$arr``[``\$i``] < ``\$arr``[``\$i` `+ 1])` `            ``swap(``\$arr``[``\$i``], ``\$arr``[``\$i` `+ 1]);` `    ``}` `}`   `/* Utility that prints out ` `an array in a line */` `function` `printArray(&``\$arr``, ``\$size``)` `{` `    ``for` `(``\$i` `= 0; ``\$i` `< ``\$size``; ``\$i``++)` `        ``echo` `\$arr``[``\$i``] . ``" "``;`   `    ``echo` `"\n"``;` `}`   `// Driver Code` `\$arr` `= ``array``(6, 4, 2, 1, 8, 3 );` `\$n` `= sizeof(``\$arr``);`   `echo` `"Before rearranging: \n"``;` `printArray(``\$arr``, ``\$n``);`   `rearrange(``\$arr``, ``\$n``);`   `echo` `"After rearranging: \n"``;` `printArray(``\$arr``, ``\$n``);`   `// This code is contributed ` `// by ChitraNayal` `?>`

## Javascript

 ``

Output:

```Before rearranging:
6 4 2 1 8 3
After rearranging:
4 6 1 8 2 3```

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 Sukanta Nandi. 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.