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# Java Program for Block swap algorithm for array rotation

Write a function rotate(ar[], d, n) that rotates arr[] of size n by d elements. Rotation of the above array by 2 will make array 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:

## Java

 `import` `java.util.*;` `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);    ``        ``}``        ``else` `/* If B is shorter*/`   `        ``{``            ``swap(arr, i, d, n - d);``            ``leftRotateRec(arr, n - d + i, ``2` `* d - n, d); ``/*This is tricky*/``        ``}``    ``}` `/*UTILITY FUNCTIONS*/``/* function to print an array */``public` `static` `void` `printArray(``int` `arr[], ``int` `size)``{``    ``int` `i;``    ``for``(i = ``0``; i < size; i++)``        ``System.out.print(arr[i] + ``" "``);``    ``System.out.println();``}` `/*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 codeseeker`

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.

## Java

 `//Java 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;``    ``}``    ``// printArray(arr, 7);``}``/*Finally, block swap A and B*/``swap(arr, d-i, d, i);``}`

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/

References:
http://www.cs.bell-labs.com/cm/cs/pearls/s02b.pdf
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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