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

Given an array, rearrange the array such that :

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

**Note :** There can be multiple answers.**Examples:**

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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 elementsNote :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}

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 <iostream>` `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[0]);` ` ` `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

`<?php` `// PHP code to rearrange an array such` `// that even index elements are smaller` `// and odd index elements are greater` `// than their next.` `function` `swap(&` `$a` `, &` `$b` `)` `{` ` ` `$temp` `= ` `$a` `;` ` ` `$a` `= ` `$b` `;` ` ` `$b` `= ` `$temp` `;` `}` `function` `rearrange(&` `$arr` `, ` `$n` `)` `{` ` ` `for` `(` `$i` `= 0; ` `$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 */` `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

`<script>` `// JavaScript code to rearrange an array such that` `// even index elements are smaller and odd` `// index elements are greater than their` `// next.` `let rearrange = (arr, n)=>{` ` ` `for` `(let i = 0; i < n - 1; i++) {` ` ` `if` `(i % 2 == 0 && arr[i] > arr[i + 1]){` ` ` `// swap(arr[i], arr[i + 1]);` ` ` `let temp = arr[i];` ` ` `arr[i] = arr[i+1];` ` ` `arr[i+1] = temp;` ` ` `}` ` ` `if` `(i % 2 != 0 && arr[i] < arr[i + 1]){` ` ` `// swap(arr[i], arr[i + 1]);` ` ` `let temp = arr[i];` ` ` `arr[i] = arr[i+1];` ` ` `arr[i+1] = temp;` ` ` `}` ` ` `}` `}` `// function to print array` `let printArray = (arr, size)=>{` ` ` `ans = ` `''` `;` ` ` `for` `(let i = 0; i < size; i++){` ` ` `ans += arr[i] + ` `" "` `;` ` ` `}` ` ` `document.write(ans);` `}` `// Driver Code` `let arr = [ 6, 4, 2, 1, 8, 3 ];` `let n = arr.length;` `document.write(` `"Before rearranging: "` `);` `printArray(arr, n);` `rearrange(arr, n);` `document.write(` `"<br>"` `);` `document.write(` `"After rearranging: "` `);` `printArray(arr, n);` `</script>` |

**Output:**

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

**Time Complexity :**O(n)

**Auxiliary Space: **O(1)**Reference :**

discuss.codechef.com/questions/43062/anuund-editorial

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