# Reversal algorithm for array rotation

• Difficulty Level : Easy
• Last Updated : 10 Mar, 2022

Write a function rotate(arr[], d, n) that rotates arr[] of size n by d elements.
Example :

```Input :  arr[] = [1, 2, 3, 4, 5, 6, 7]
d = 2
Output : arr[] = [3, 4, 5, 6, 7, 1, 2] ```

Rotation of the above array by 2 will make array

The first 3 methods to rotate an array by d elements has been discussed in this post.
Method 4 (The Reversal Algorithm) :
Algorithm :

```rotate(arr[], d, n)
reverse(arr[], 1, d) ;
reverse(arr[], d + 1, n);
reverse(arr[], 1, n);```

Let AB are the two parts of the input array where A = arr[0..d-1] and B = arr[d..n-1]. The idea of the algorithm is :

• Reverse A to get ArB, where Ar is reverse of A.
• Reverse B to get ArBr, where Br is reverse of B.
• Reverse all to get (ArBr) r = BA.

Example :
Let the array be arr[] = [1, 2, 3, 4, 5, 6, 7], d =2 and n = 7
A = [1, 2] and B = [3, 4, 5, 6, 7]

• Reverse A, we get ArB = [2, 1, 3, 4, 5, 6, 7]
• Reverse B, we get ArBr = [2, 1, 7, 6, 5, 4, 3]
• Reverse all, we get (ArBr)r = [3, 4, 5, 6, 7, 1, 2]

Below is the implementation of the above approach :

## C++

 `// C++ program for reversal algorithm``// of array rotation``#include ``using` `namespace` `std;` `/*Function to reverse arr[] from index start to end*/``void` `reverseArray(``int` `arr[], ``int` `start, ``int` `end)``{``    ``while` `(start < end) {``        ``int` `temp = arr[start];``        ``arr[start] = arr[end];``        ``arr[end] = temp;``        ``start++;``        ``end--;``    ``}``}` `/* Function to left rotate arr[] of size n by d */``void` `leftRotate(``int` `arr[], ``int` `d, ``int` `n)``{``    ``if` `(d == 0)``        ``return``;``    ``// in case the rotating factor is``    ``// greater than array length``    ``d = d % n;` `    ``reverseArray(arr, 0, d - 1);``    ``reverseArray(arr, d, n - 1);``    ``reverseArray(arr, 0, n - 1);``}` `// Function to print an array``void` `printArray(``int` `arr[], ``int` `size)``{``    ``for` `(``int` `i = 0; i < size; i++)``        ``cout << arr[i] << ``" "``;``}` `/* Driver program to test above functions */``int` `main()``{``    ``int` `arr[] = { 1, 2, 3, 4, 5, 6, 7 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``int` `d = 2;` `    ``// Function calling``    ``leftRotate(arr, d, n);``    ``printArray(arr, n);` `    ``return` `0;``}`

## C

 `// C/C++ program for reversal algorithm of array rotation``#include ` `/*Utility function to print an array */``void` `printArray(``int` `arr[], ``int` `size);` `/* Utility function to reverse arr[] from start to end */``void` `reverseArray(``int` `arr[], ``int` `start, ``int` `end);` `/* Function to left rotate arr[] of size n by d */``void` `leftRotate(``int` `arr[], ``int` `d, ``int` `n)``{` `    ``if` `(d == 0)``        ``return``;``    ``// in case the rotating factor is``    ``// greater than array length``    ``d = d % n;` `    ``reverseArray(arr, 0, d - 1);``    ``reverseArray(arr, d, n - 1);``    ``reverseArray(arr, 0, n - 1);``}` `/*UTILITY FUNCTIONS*/``/* function to print an array */``void` `printArray(``int` `arr[], ``int` `size)``{``    ``int` `i;``    ``for` `(i = 0; i < size; i++)``        ``printf``(``"%d "``, arr[i]);``}` `/*Function to reverse arr[] from index start to end*/``void` `reverseArray(``int` `arr[], ``int` `start, ``int` `end)``{``    ``int` `temp;``    ``while` `(start < end) {``        ``temp = arr[start];``        ``arr[start] = arr[end];``        ``arr[end] = temp;``        ``start++;``        ``end--;``    ``}``}` `/* Driver program to test above functions */``int` `main()``{``    ``int` `arr[] = { 1, 2, 3, 4, 5, 6, 7 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);``    ``int` `d = 2;` `    ``leftRotate(arr, d, n);``    ``printArray(arr, n);``    ``return` `0;``}`

## Java

 `// Java program for reversal algorithm of array rotation``import` `java.io.*;` `class` `LeftRotate {``    ``/* Function to left rotate arr[] of size n by d */``    ``static` `void` `leftRotate(``int` `arr[], ``int` `d)``    ``{` `        ``if` `(d == ``0``)``            ``return``;` `        ``int` `n = arr.length;``        ``// in case the rotating factor is``        ``// greater than array length``        ``d = d % n;``        ``reverseArray(arr, ``0``, d - ``1``);``        ``reverseArray(arr, d, n - ``1``);``        ``reverseArray(arr, ``0``, n - ``1``);``    ``}` `    ``/*Function to reverse arr[] from index start to end*/``    ``static` `void` `reverseArray(``int` `arr[], ``int` `start, ``int` `end)``    ``{``        ``int` `temp;``        ``while` `(start < end) {``            ``temp = arr[start];``            ``arr[start] = arr[end];``            ``arr[end] = temp;``            ``start++;``            ``end--;``        ``}``    ``}` `    ``/*UTILITY FUNCTIONS*/``    ``/* function to print an array */``    ``static` `void` `printArray(``int` `arr[])``    ``{``        ``for` `(``int` `i = ``0``; i < arr.length; i++)``            ``System.out.print(arr[i] + ``" "``);``    ``}` `    ``/* Driver program to test above functions */``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int` `arr[] = { ``1``, ``2``, ``3``, ``4``, ``5``, ``6``, ``7` `};``        ``int` `n = arr.length;``        ``int` `d = ``2``;` `        ``leftRotate(arr, d); ``// Rotate array by d``        ``printArray(arr);``    ``}``}``/*This code is contributed by Devesh Agrawal*/`

## Python3

 `# Python program for reversal algorithm of array rotation` `# Function to reverse arr[] from index start to end`  `def` `reverseArray(arr, start, end):``    ``while` `(start < end):``        ``temp ``=` `arr[start]``        ``arr[start] ``=` `arr[end]``        ``arr[end] ``=` `temp``        ``start ``+``=` `1``        ``end ``=` `end``-``1` `# Function to left rotate arr[] of size n by d`  `def` `leftRotate(arr, d):` `    ``if` `d ``=``=` `0``:``        ``return``    ``n ``=` `len``(arr)``    ``# in case the rotating factor is``    ``# greater than array length``    ``d ``=` `d ``%` `n``    ``reverseArray(arr, ``0``, d``-``1``)``    ``reverseArray(arr, d, n``-``1``)``    ``reverseArray(arr, ``0``, n``-``1``)` `# Function to print an array`  `def` `printArray(arr):``    ``for` `i ``in` `range``(``0``, ``len``(arr)):``        ``print` `(arr[i],end``=``' '``)`  `# Driver function to test above functions``arr ``=` `[``1``, ``2``, ``3``, ``4``, ``5``, ``6``, ``7``]``n ``=` `len``(arr)``d ``=` `2` `leftRotate(arr, d)  ``# Rotate array by 2``printArray(arr)` `# This code is contributed by Devesh Agrawal`

## C#

 `// C# program for reversal algorithm``// of array rotation``using` `System;` `class` `GFG {``    ``/* Function to left rotate arr[]``    ``of size n by d */``    ``static` `void` `leftRotate(``int``[] arr, ``int` `d)``    ``{` `        ``if` `(d == 0)``            ``return``;``        ``int` `n = arr.Length;``          ``// in case the rotating factor is``        ``// greater than array length``        ``d = d % n;``        ``reverseArray(arr, 0, d - 1);``        ``reverseArray(arr, d, n - 1);``        ``reverseArray(arr, 0, n - 1);``    ``}` `    ``/* Function to reverse arr[] from``    ``index start to end*/``    ``static` `void` `reverseArray(``int``[] arr, ``int` `start,``                             ``int` `end)``    ``{``        ``int` `temp;``        ``while` `(start < end) {``            ``temp = arr[start];``            ``arr[start] = arr[end];``            ``arr[end] = temp;``            ``start++;``            ``end--;``        ``}``    ``}` `    ``/*UTILITY FUNCTIONS*/``    ``/* function to print an array */``    ``static` `void` `printArray(``int``[] arr)``    ``{``        ``for` `(``int` `i = 0; i < arr.Length; i++)``            ``Console.Write(arr[i] + ``" "``);``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``int``[] arr = { 1, 2, 3, 4, 5, 6, 7 };``        ``int` `n = arr.Length;``        ``int` `d = 2;` `        ``leftRotate(arr, d); ``// Rotate array by 2``        ``printArray(arr);``    ``}``}` `// This code is contributed by Sam007`

## PHP

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## Javascript

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Output :

`3 4 5 6 7 1 2`

Time Complexity : O(n)

Auxiliary Space: O(1)