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] 


 

Array

Rotation of the above array by 2 will make array
 

ArrayRotation1



 

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++

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// C++ program for reversal algorithm
// of array rotation
#include <bits/stdc++.h>
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;
}

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C

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// C/C++ program for reversal algorithm of array rotation
#include <stdio.h>
 
/*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;
}

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Java

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// 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*/

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Python

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# 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],
 
 
# 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

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C#

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// 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

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PHP

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<?php
// PHP program for reversal
// algorithm of array rotation
 
/* Function to left rotate
   arr of size n by d */
function leftRotate(&$arr, $d, $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 reverse $arr
  from index start to end*/
function reverseArray(&$arr,
                       $start, $end)
{
    while ($start < $end)
    {
        $temp = $arr[$start];
        $arr[$start] = $arr[$end];
        $arr[$end] = $temp;
        $start++;
        $end--;
    }
}
 
// Function to
// print an array
function printArray($arr, $size)
{
    for ($i = 0; $i < $size; $i++)
    print $arr[$i]." ";
}
 
// Driver code
$arr = array(1, 2, 3,
             4, 5, 6, 7);
$n = sizeof($arr);
$d = 2;
 
// Function calling
leftRotate($arr, $d, $n);
printArray($arr, $n);
     
// This code is contributed
// by ChitraNayal
?>

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Output : 

3 4 5 6 7 1 2

Time Complexity : O(n)
 

https://www.youtube.com/watch?v=QOpU9-l5T7Y 
 

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