# Reversal algorithm for array rotation

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

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 rvereseArray(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; rvereseArray(arr, 0, d - 1); rvereseArray(arr, d, n - 1); rvereseArray(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); int d = 2; // in case the rotating factor is // greater than array length d = d % n; // Function calling leftRotate(arr, d, n); printArray(arr, n); return 0; } [tabby title="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 rvereseArray(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;
rvereseArray(arr, 0, d – 1);
rvereseArray(arr, d, n – 1);
rvereseArray(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 rvereseArray(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); int d = 2; // in case the rotating factor is // greater than array length d = d % n; leftRotate(arr, d, n); printArray(arr, n); return 0; } [tabby title="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; rvereseArray(arr, 0, d - 1); rvereseArray(arr, d, n - 1); rvereseArray(arr, 0, n - 1); } /*Function to reverse arr[] from index start to end*/ static void rvereseArray(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; // in case the rotating factor is // greater than array length d = d % n; leftRotate(arr, d); // Rotate array by d printArray(arr); } } /*This code is contributed by Devesh Agrawal*/ [tabby title="Python"] # Python program for reversal algorithm of array rotation # Function to reverse arr[] from index start to end def rverseArray(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) rverseArray(arr, 0, d-1) rverseArray(arr, d, n-1) rverseArray(arr, 0, n-1) # Function to print an array def printArray(arr): for i in range(0, len(arr)): print arr[i], # Driver function to test above functions arr = [1, 2, 3, 4, 5, 6, 7] n = len(arr) d = 2 # in case the rotating factor is # greater than array length d = d % n leftRotate(arr, d) # Rotate array by 2 printArray(arr) # This code is contributed by Devesh Agrawal [tabby title="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; rvereseArray(arr, 0, d - 1); rvereseArray(arr, d, n - 1); rvereseArray(arr, 0, n - 1); } /* Function to reverse arr[] from index start to end*/ static void rvereseArray(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; // in case the rotating factor is // greater than array length d = d % n; leftRotate(arr, 2); // Rotate array by 2 printArray(arr); } } // This code is contributed by Sam007 [tabby title="PHP"]

Output :

`3 4 5 6 7 1 2`

Time Complexity : O(n)

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