# Shuffle array {a1, a2, .. an, b1, b2, .. bn} as {a1, b1, a2, b2, a3, b3, ……, an, bn} without using extra space

Given an array of 2n elements in the following format { a1, a2, a3, a4, ….., an, b1, b2, b3, b4, …., bn }. The task is shuffle the array to {a1, b1, a2, b2, a3, b3, ……, an, bn } without using extra space.

Examples :

```Input : arr[] = { 1, 2, 9, 15 }
Output : 1 9 2 15

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

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

We have discussed O(n2) and O(n Log n) solutions of this problem in below post.

Shuffle 2n integers in format {a1, b1, a2, b2, a3, b3, ……, an, bn} without using extra space

Here a new solution is discussed that works in linear time. We know that the first and last numbers don’t move from their places. And, we keep track of the index from which any number is picked and where the target index is. We know that, if we’re picking ai, it has to go to the index 2 * i – 1 and if bi, it has to go 2 * i. We can check from where we have picked a certain number based on the picking index if it greater or less than n.

We will have to do this for 2 * n – 2 times, assuming that n = half of length of array.
We, get two cases, when n is even and odd, hence we initialize appropriately the start variable.
Note : Indexes are considered 1 based in array for simplicity.

## C++

 `// C++ program to shuffle an array in O(n) time ` `// and O(1) extra space ` `#include ` `using` `namespace` `std; ` ` `  `// Shuffles an array of size 2n. Indexes ` `// are considered starting from 1. ` `void` `shufleArray(``int` `a[], ``int` `n) ` `{ ` `    ``n = n / 2; ` ` `  `    ``for` `(``int` `start = n + 1, j = n + 1, done = 0, i;  ` `                      ``done < 2 * n - 2; done++) { ` `        ``if` `(start == j) { ` `            ``start--; ` `            ``j--; ` `        ``} ` ` `  `        ``i = j > n ? j - n : j; ` `        ``j = j > n ? 2 * i : 2 * i - 1; ` ` `  `        ``swap(a[start], a[j]); ` `    ``} ` `} ` ` `  `// Driven Program ` `int` `main() ` `{ ` `    ``// The first element is bogus. We have used ` `    ``// one based indexing for simplicity. ` `    ``int` `a[] = { -1, 1, 3, 5, 7, 2, 4, 6, 8 }; ` `    ``int` `n = ``sizeof``(a) / ``sizeof``(a); ` ` `  `    ``shufleArray(a, n); ` ` `  `    ``for` `(``int` `i = 1; i < n; i++) ` `        ``cout << a[i] << ``" "``; ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java program to shuffle an  ` `// array in O(n) time and O(1) ` `// extra space ` `import` `java.io.*; ` ` `  `public` `class` `GFG { ` `     `  `// Shuffles an array of size 2n. ` `// Indexes are considered starting ` `// from 1. ` `static` `void` `shufleArray(``int` `[]a, ``int` `n) ` `{ ` `    ``int` `temp; ` `    ``n = n / ``2``; ` ` `  `    ``for` `(``int` `start = n + ``1``, j = n + ``1``, done = ``0``, i; ` `         ``done < ``2` `* n - ``2``; done++) ` `    ``{ ` `        ``if` `(start == j) ` `        ``{ ` `            ``start--; ` `            ``j--; ` `        ``} ` ` `  `        ``i = j > n ? j - n : j; ` `        ``j = j > n ? ``2` `* i : ``2` `* i - ``1``; ` `        ``temp= a[start]; ` `        ``a[start]= a[j]; ` `        ``a[j]= temp; ` ` `  `    ``} ` `} ` ` `  `    ``// Driver code ` `    ``static` `public` `void` `main (String[] args) ` `    ``{ ` `            ``// The first element is bogus. We have used ` `            ``// one based indexing for simplicity. ` `            ``int` `[]a = {-``1``, ``1``, ``3``, ``5``, ``7``, ``2``, ``4``, ``6``, ``8``}; ` `            ``int` `n = a.length; ` `         `  `            ``shufleArray(a, n); ` `         `  `            ``for` `(``int` `i = ``1``; i < n; i++) ` `                ``System.out.print(a[i] +``" "``); ` `                 `  `    ``} ` `} ` ` `  `// This Code is contributed by vt_m. `

## Python 3

 `# Python 3 program to shuffle an array  ` `# in O(n) time and O(1) extra space ` ` `  `# Shuffles an array of size 2n. Indexes ` `# are considered starting from 1. ` `def` `shufleArray(a, n): ` `     `  `    ``n ``=` `n ``/``/` `2` ` `  `    ``start ``=` `n ``+` `1` `    ``j ``=` `n ``+` `1` `    ``for` `done ``in` `range``( ``2` `*` `n ``-` `2``) : ` `        ``if` `(start ``=``=` `j) : ` `            ``start ``-``=` `1` `            ``j ``-``=` `1` ` `  `        ``i ``=` `j ``-` `n ``if` `j > n ``else` `j ` `        ``j ``=` `2` `*` `i ``if` `j > n ``else` `2` `*` `i ``-` `1` ` `  `        ``a[start], a[j] ``=` `a[j], a[start] ` ` `  `# Driver Code ` `if` `__name__ ``=``=` `"__main__"``: ` `     `  `    ``# The first element is bogus. We have used ` `    ``# one based indexing for simplicity. ` `    ``a ``=` `[ ``-``1``, ``1``, ``3``, ``5``, ``7``, ``2``, ``4``, ``6``, ``8` `] ` `    ``n ``=` `len``(a) ` ` `  `    ``shufleArray(a, n) ` ` `  `    ``for` `i ``in` `range``(``1``, n): ` `        ``print``(a[i], end ``=` `" "``) ` ` `  `# This code is contributed  ` `# by ChitraNayal `

## C#

 `// C# program to shuffle an  ` `// array in O(n) time and O(1) ` `// extra space ` `using` `System; ` ` `  `public` `class` `GFG { ` `     `  `// Shuffles an array of size 2n. ` `// Indexes are considered starting ` `// from 1. ` `static` `void` `shufleArray(``int` `[]a, ``int` `n) ` `{ ` `    ``int` `temp; ` `    ``n = n / 2; ` ` `  `    ``for` `(``int` `start = n + 1, j = n + 1, done = 0, i; ` `         ``done < 2 * n - 2; done++) ` `    ``{ ` `        ``if` `(start == j) ` `        ``{ ` `            ``start--; ` `            ``j--; ` `        ``} ` ` `  `        ``i = j > n ? j - n : j; ` `        ``j = j > n ? 2 * i : 2 * i - 1; ` `        ``temp= a[start]; ` `        ``a[start]= a[j]; ` `        ``a[j]= temp; ` ` `  `    ``} ` `} ` ` `  `    ``// Driven code ` `    ``static` `public` `void` `Main (String[] args) ` `    ``{ ` `            ``// The first element is bogus. We have used ` `            ``// one based indexing for simplicity. ` `            ``int` `[]a = {-1, 1, 3, 5, 7, 2, 4, 6, 8}; ` `            ``int` `n = a.Length; ` `         `  `            ``shufleArray(a, n); ` `         `  `            ``for` `(``int` `i = 1; i < n; i++) ` `                ``Console.Write(a[i] +``" "``); ` `                 `  `    ``} ` `} ` ` `  `// This Code is contributed by vt_m. `

## PHP

 ` ``\$k` `? ``\$j` `- ``\$k` `: ``\$j``; ` `        ``\$j` `= ``\$j` `> ``\$k` `? 2 * ``\$i` `: 2 * ``\$i` `- 1; ` `        ``\$temp` `= ``\$a``[``\$start``]; ` `        ``\$a``[``\$start``] = ``\$a``[``\$j``]; ` `        ``\$a``[``\$j``] = ``\$temp``; ` ` `  `    ``} ` `    ``for` `(``\$i` `= 1; ``\$i` `< ``\$n``; ``\$i``++) ` `                ``echo` `\$a``[``\$i``] . ``" "``; ` `} ` ` `  `// Driver code ` ` `  `// The first element is bogus.  ` `// We have used one based  ` `// indexing for simplicity. ` `\$a` `= ``array``(-1, 1, 3, 5, 7, 2, 4, 6, 8); ` `\$n` `= ``count``(``\$a``); ` `shufleArray(``\$a``, ``\$n``); ` `     `  `// This code is contributed by Sam007 ` `?> `

Output :

```1 2 3 4 5 6 7 8
```

This article is contributed by azam58. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

My Personal Notes arrow_drop_up

Improved By : vt_m, Sam007, chitranayal

Article Tags :
Practice Tags :

Be the First to upvote.

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.