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

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

Note : Another valid answer
is arr[] = {3, 4, 2, 5}

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

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

This problem is similar to sort an array in wave form.

A simple solution is to sort the array in decreasing order, then starting from 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}

## 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: "````; ` `    ``printArray(arr, n); ` ` `  `    ``rearrange(arr, n); ` ` `  `    ``cout << ````"After rearranging: "````; ` `    ``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: "````); ` `        ``printArray(arr, n); ` ` `  `        ``rearrange(arr, n); ` ` `  `        ``System.out.print(````"After rearranging: "````); ` `        ``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` ```" "````; ` `} ` ` `  `// Driver Code ` `\$arr` `= ``array``(6, 4, 2, 1, 8, 3 ); ` `\$n` `= sizeof(``\$arr``); ` ` `  `echo` ```"Before rearranging: "````; ` `printArray(``\$arr``, ``\$n``); ` ` `  `rearrange(``\$arr``, ``\$n``); ` ` `  `echo` ```"After rearranging: "````; ` `printArray(``\$arr``, ``\$n``); ` ` `  `// This code is contributed  ` `// by ChitraNayal ` `?> `

Output:

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

Time Complexity :O(n)

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

This article is attributed to GeeksforGeeks.org

## tags:

Arrays array-rearrange Arrays

code

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