C# Program for Block swap algorithm for array rotation

Last Updated : 19 Sep, 2023

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

Array

Rotation of the above array by 2 will make array

ArrayRotation1

Algorithm :

Initialize A = arr[0..d-1] and B = arr[d..n-1]
1) Do following until size of A is equal to size of B

  a)  If A is shorter, divide B into Bl and Br such that Br is of same 
       length as A. Swap A and Br to change ABlBr into BrBlA. Now A
       is at its final place, so recur on pieces of B.  

   b)  If A is longer, divide A into Al and Ar such that Al is of same 
       length as B Swap Al and B to change AlArB into BArAl. Now B
       is at its final place, so recur on pieces of A.

2)  Finally when A and B are of equal size, block swap them.

Recursive Implementation:

C#

using System; 
  
class GFG{ 
      
// Wrapper over the recursive function 
// leftRotateRec()  
// It left rotates []arr by d. 
public static void leftRotate(int []arr,  
                              int d, int n) 
{ 
    leftRotateRec(arr, 0, d, n); 
} 
  
public static void leftRotateRec(int []arr, int i,  
                                 int d, int n) 
{ 
      
    // Return If number of elements  
    // to be rotated is zero or equal 
    // to array size 
    if(d == 0 || d == n)  
        return;  
      
    // If number of elements to be rotated  
    // is exactly half of array size  
    if(n - d == d)  
    {  
        swap(arr, i, n - d + i, d);  
        return;  
    }  
      
    // If A is shorter 
    if(d < n - d)  
    {  
        swap(arr, i, n - d + i, d);  
        leftRotateRec(arr, i, d, n - d);      
    }  
      
    // If B is shorter 
    else 
    {  
        swap(arr, i, d, n - d);  
          
        // This is tricky 
        leftRotateRec(arr, n - d + i,  
                       2 * d - n, d);  
    }  
} 
  
// UTILITY FUNCTIONS 
// Function to print an array  
public static void printArray(int []arr, 
                              int size)  
{  
    int i;  
    for(i = 0; i < size; i++)  
        Console.Write(arr[i] + " ");  
          
    Console.WriteLine();  
}  
  
// This function swaps d elements  
// starting at index fi with d elements 
// starting at index si  
public static void swap(int []arr, int fi, 
                        int si, int d)  
{  
    int i, temp;  
    for(i = 0; i < d; i++)  
    {  
        temp = arr[fi + i];  
        arr[fi + i] = arr[si + i];  
        arr[si + i] = temp;  
    }  
}  
  
// Driver Code 
public static void Main(String[] args)  
{ 
    int []arr = { 1, 2, 3, 4, 5, 6, 7 };  
      
    leftRotate(arr, 2, 7);  
    printArray(arr, 7);      
} 
} 
  
// This code is contributed by amal kumar choubey 

Output:

3 5 4 6 7 1 2

Time Complexity: O(N), where N represents the size of the given array.
Auxiliary Space: O(N), due to recursive stack space.

Iterative Implementation:
Here is iterative implementation of the same algorithm. Same utility function swap() is used here.

C#

// C# code for above implementation 
static void leftRotate(int []arr, int d, int n) 
{ 
    int i, j; 
    if(d == 0 || d == n) 
        return; 
    i = d; 
    j = n - d; 
    while (i != j) 
    { 
        if(i < j) /*A is shorter*/
        { 
            swap(arr, d-i, d+j-i, i); 
            j -= i; 
        } 
        else /*B is shorter*/
        { 
            swap(arr, d-i, d, j); 
            i -= j; 
        } 
          
    } 
      
    /*Finally, block swap A and B*/
    swap(arr, d-i, d, i); 
} 
  
// This code is contributed by Rajput-Ji 

Time Complexity: O(N), where N represents the size of the given array.
Auxiliary Space: O(1), no extra space is required, so it is a constant.

Please see following posts for other methods of array rotation:
https://www.geeksforgeeks.org/array-rotation/
https://www.geeksforgeeks.org/program-for-array-rotation-continued-reversal-algorithm/

Please write comments if you find any bug in the above programs/algorithms or want to share any additional information about the block swap algorithm.

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

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