# Rearrange an array such that every odd indexed element is greater than it previous

Given an unsorted array, rearrange the array such that number at the odd index is greater than the number at the previous even index. There may be multiple outputs, we need to print one of them.

Note : Indexing is based on array so it always starts from 0.

Examples:

```Input  : arr[] = {5, 2, 3, 4}
Output : arr[] = {2, 5, 3, 4}

Input  : arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9}
Output : arr[] = {1, 3, 2, 5, 4, 7, 6, 9, 8}
```

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

If the given array has even length, then it can be done in single run. Just for each pair of neighbours, put max in odd position and min in even. So we just start cycle from beginning and compare each two elements. Let’s say
6, 5, 4, 3, 2, 1 will give us 5, 6, 3, 4, 1, 2
If array has odd length, like 6, 5, 4, 3, 2, 1, 100 then it’s slightly trickier, we need to run second, backward cycle again, but starting from the last element. So, 6, 5, 4, 3, 2, 1, 100 after first cycle will be 5, 6, 3, 4, 1, 2, 100 and after second cycle 5, 6, 3, 4, 1, 100, 2

## C++

 `// C++ program to rearrange array such that ` `// odd indexed elements are greater. ` `#include ` `using` `namespace` `std; ` ` `  `void` `rearrange(``int` `arr[], ``int` `n) ` `{ ` `    ``// Common code for odd and even lengths ` `    ``for` `(``int` `i=0; i arr[i+1]) ` `            ``swap(arr[i], arr[i+1]); ` `    ``} ` ` `  `    ``// If length is odd ` `    ``if` `(n & 1) ` `    ``{ ` `        ``for` `(``int` `i=n-1; i>0; i=i-2) ` `            ``if` `(arr[i] > arr[i-1]) ` `                ``swap(arr[i], arr[i-1]); ` `    ``} ` `} ` ` `  `/* Utility that prints out an array on a line */` `void` `printArray(``int` `arr[], ``int` `size) ` `{ ` `    ``for` `(``int` `i=0; i < size; i++) ` `        ``printf``(``"%d "``, arr[i]); ` ` `  `    ``printf``(``"\n"``); ` `} ` ` `  `/* Driver function to test above functions */` `int` `main() ` `{ ` `    ``int` `arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9}; ` `    ``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 program to rearrange array such that ` `// odd indexed elements are greater. ` ` `  `import` `java.util.Arrays; ` ` `  `class` `GFG ` `{ ` `    ``// Utitlity function to Swap two variables ` `    ``public` `static` `void` `swap(``int``[] arr, ``int` `i, ``int` `j) ` `    ``{ ` `        ``int` `temp = arr[i]; ` `        ``arr[i] = arr[j]; ` `        ``arr[j] = temp; ` `    ``} ` ` `  `    ``// method to rearrange array such that ` `    ``// odd indexed elements are greater. ` `    ``public` `static` `void` `Rearrage(``int``[] arr, ``int` `n) ` `    ``{ ` `        ``// Common code for odd and even lengths ` `        ``for` `(``int` `i = ``0``; i < n - ``1``; i = i + ``2``) { ` `            ``if` `(arr[i] > arr[i + ``1``]) ` `                ``swap(arr, i, i + ``1``); ` `        ``} ` ` `  `        ``// If length is odd ` `        ``if` `((n & ``1``) > ``0``) { ` `            ``for` `(``int` `i = n - ``1``; i > ``0``; i = i - ``2``) ` `                ``if` `(arr[i] > arr[i - ``1``]) ` `                    ``swap(arr, i, i - ``1``); ` `        ``} ` `    ``} ` ` `  `    ``// Driver Method ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int``[] arr = { ``1``, ``2``, ``3``, ``4``, ``5``, ``6``, ``7``, ``8``, ``9` `}; ` ` `  `        ``System.out.println(``"Before rearranging"``); ` `        ``System.out.println(Arrays.toString(arr)); ` ` `  `        ``Rearrage(arr, arr.length);         ` ` `  `        ``System.out.println(``"After rearranging"``); ` `        ``System.out.println(Arrays.toString(arr)); ` `    ``} ` `} ` `/* This code is contributed by Mr. Somesh Awasthi */`

## Python3

 `# Python 3 program to rearrange array such  ` `# that odd indexed elements are greater. ` `def` `rearrange(arr, n): ` `     `  `    ``# Common code for odd and even lengths ` `    ``for` `i ``in` `range``(``0``, n ``-` `1``, ``2``): ` `        ``if` `(arr[i] > arr[i ``+` `1``]): ` `            ``temp ``=` `arr[i] ` `            ``arr[i] ``=` `arr[i ``+` `1``] ` `            ``arr[i ``+` `1``] ``=` `temp ` ` `  `    ``# If length is odd ` `    ``if` `(n & ``1``): ` `        ``i ``=` `n ``-` `1` `        ``while``(i > ``0``): ` `            ``if` `(arr[i] > arr[i ``-` `1``]): ` `                ``temp ``=` `arr[i] ` `                ``arr[i] ``=` `arr[i ``-` `1``] ` `                ``arr[i ``-` `1``] ``=` `temp ` `            ``i ``-``=` `2` ` `  `# Utility that prints out an  ` `# array on a line  ` `def` `printArray(arr, size): ` `    ``for` `i ``in` `range``(``0``, size, ``1``): ` `        ``print``(arr[i], end ``=` `" "``) ` `    ``print``(``"\n"``) ` `     `  `# Driver Code ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``arr ``=` `[``1``, ``2``, ``3``, ``4``, ``5``, ``6``, ``7``, ``8``, ``9``] ` `    ``n ``=` `len``(arr) ` `    ``print``(``"Before rearranging"``) ` `     `  `    ``printArray(arr, n) ` `    ``rearrange(arr, n) ` `    ``print``(``"After rearranging"``) ` `     `  `    ``printArray(arr, n) ` ` `  `# This code is contributed by ` `# Surendra_Gangwar `

## C#

 `// C# program to rearrange  ` `// array such that odd  ` `// indexed elements are  ` `// greater. ` `using` `System; ` ` `  `class` `GFG ` `{ ` ` `  `// Utitlity function to ` `// Swap two variables ` `public` `static` `void` `swap(``int``[] arr,  ` `                        ``int` `i, ``int` `j) ` `{ ` `    ``int` `temp = arr[i]; ` `    ``arr[i] = arr[j]; ` `    ``arr[j] = temp; ` `} ` ` `  `// method to rearrange  ` `// array such that odd  ` `// indexed elements are  ` `// greater. ` `public` `static` `void` `Rearrage(``int``[] arr,  ` `                            ``int` `n) ` `{ ` `    ``// Common code for odd  ` `    ``// and even lengths ` `    ``for` `(``int` `i = 0; i < n - 1; i = i + 2) ` `    ``{ ` `        ``if` `(arr[i] > arr[i + 1]) ` `            ``swap(arr, i, i + 1); ` `    ``} ` ` `  `    ``// If length is odd ` `    ``if` `((n & 1) > 0) ` `    ``{ ` `        ``for` `(``int` `i = n - 1;  ` `                 ``i > 0; i = i - 2) ` `            ``if` `(arr[i] > arr[i - 1]) ` `                ``swap(arr, i, i - 1); ` `    ``} ` `} ` ` `  `// Driver Code ` `static` `void` `Main() ` `{ ` `    ``int``[] arr = {1, 2, 3, 4,  ` `                 ``5, 6, 7, 8, 9}; ` ` `  `    ``Console.WriteLine(``"Before rearranging"``); ` `    ``for``(``int` `i = 0; i < arr.Length; i++) ` `        ``Console.Write(arr[i] + ``" "``); ` ` `  `    ``Rearrage(arr, arr.Length);      ` ` `  `    ``Console.WriteLine(``"\nAfter rearranging"``); ` `    ``for``(``int` `i = 0; i < arr.Length; i++) ` `        ``Console.Write(arr[i] + ``" "``); ` `} ` `} ` ` `  `// This code is contributed ` `// by Sam007 `

## PHP

 ` ``\$arr``[``\$i` `+ 1]) ` `        ``{ ` `            ``\$temp` `= ``\$arr``[``\$i``]; ` `            ``\$arr``[``\$i``] = ``\$arr``[``\$i` `+ 1]; ` `            ``\$arr``[``\$i` `+ 1] = ``\$temp``; ` `        ``} ` `    ``} ` ` `  `    ``// If length is odd ` `    ``if` `(``\$n` `& 1) ` `    ``{ ` `        ``for` `(``\$i` `= ``\$n` `- 1; ``\$i` `> 0; ``\$i` `= ``\$i` `- 2) ` `            ``if` `(``\$arr``[``\$i``] > ``\$arr``[``\$i` `- 1]) ` `            ``{ ` `                ``\$temp` `= ``\$arr``[``\$i``]; ` `                ``\$arr``[``\$i``] = ``\$arr``[``\$i` `- 1]; ` `                ``\$arr``[``\$i` `- 1] = ``\$temp``; ` `            ``} ` `    ``} ` `} ` ` `  `// Utility that prints out an array on a line ` `function` `printArray(&``\$arr``, ``\$size``) ` `{ ` `    ``for` `(``\$i` `= 0; ``\$i` `< ``\$size``; ``\$i``++) ` `        ``echo` `\$arr``[``\$i``] . ``" "``; ` ` `  `    ``echo` `"\n"``; ` `} ` ` `  `// Driver Code ` `\$arr` `= ``array``(1, 2, 3, 4, 5, 6, 7, 8, 9); ` `\$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 ita_c ` `?> `

Output:

```Before rearranging
1 2 3 4 5 6 7 8 9
After rearranging
1 3 2 5 4 7 6 9 8
```

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