Java Program for 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

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

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 Java implementation of the above approach:

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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)
    {
        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 };
        leftRotate(arr, 2); // Rotate array by 2
        printArray(arr);
    }
}
/*This code is contributed by Devesh Agrawal*/

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Output:

3 4 5 6 7 1 2

Time Complexity: O(n)

Please refer complete article on Reversal algorithm for array rotation for more details!



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