Block Swap Algorithm or Array Rotation in Java

It is very interesting problem frequently asked in interviews of top IT companies like Google, Amazon, TCS, Accenture, etc. By solving the problem, one wants to check the logical ability, critical thinking, and problem-solving skill of the interviewee. So, in this section, we are going to discuss block swap algorithm and array rotation in Java with different approaches and logic. Also, we will create Java programs for the same.

Array Rotation Example

Input:

int arr[] = {7, 9, 8, 0, 5, 1, 6, 4}, s = 8, d = 2

Output: result[] = {8, 0, 5, 1, 6, 4, 7, 9}

Explanation: s is the size of the input array. d = 2 means we have to rotate the array 2 times. When we rotate the first time, we get

{9, 8, 0, 5, 1, 6, 4, 7}. Now, we rotate the array a second time, and we get the following.

Let's see another example.

Input:

int arr[] = {6, 8, 7, 9, 0, 5, 1, 3, 2, 4}, s = 10, d = 5

Output: result[] = {5, 1, 2, 3, 4, 6, 8, 7, 9, 0}

Explanation: After rotating the array 5 times, we get the following:

{5, 1, 2, 3, 4, 6, 8, 7, 9, 0}, which is our answer.

Block Swap Algorithm

In this section, we have used block swap algorithm to rotate array. Observe the algorithm:

Step 1: Divide the input array into two sub-arrays with div as division point. Let them be A = arr[0 ... div - 1] and B = arr[div ... n - 1].

Step 2: Until the size of A and B are equal, follow the following steps:

If the size of A is greater than B, then divide A into two portions, A1 and A2, such that the size of A1 is equal to the size of A2. Swap the sub-arrays A1 and B. It changes the array from A1A2B to BA2A1.
If the size of B > A, divide B into two parts, B1 and B2, in such a way that the size of B2 is the same as the size of B. Swap the subarrays A and B2. It changes the array from AB1B2 to B2B1A.

Step 3: When the A and B sizes are equal, swap them.

Approach: Using Recursion

The following program uses the above-mentioned algorithm.

FileName: RotateArray.java

public class RotateArray
{
// A Wrapper method over the recursive method leftRotateRecursive()
// It rotates the array elements by r.
public void leftRotate(int arr[], int r, int s)
{
leftRotateRecursive(arr, 0, r, s);
}
public void leftRotateRecursive(int input[], int j, int r, int s)
{
// the array will remain the same when size of the 
// input array is equal to the number of times the 
// the array is rotated. Also, if we do not rotate the array 
// at all we get the same array
if(r == 0 || r == s)
{
return;
}
// If the number of elements that has to be 
// rotated is exactly half of the input array size 
if(s - r == r)
{
swap(input, j, s - r + j, r);
return;
}
// If A is short in length
if(r < s - r)
{
swap(input, j, s - r + j, r);
leftRotateRecursive(input, j, r, s - r);	
}
else // B is short in length
{
swap(input, j, r, s - r);
leftRotateRecursive(input, s - r + j, 2 * r - s, r); 
}
}
// The method swaps the r elements
// beginning at the index f with r elements
// starting at the index g
public void swap(int input[], int f, int g, int r)
{
int k;
int temp;
for(k = 0; k < r; k++)
{
temp = input[f + k];
input[f + k] = input[g + k];
input[g + k] = temp;
}
}
// main method
public static void main (String[] argvs)
{
// creating an object of the class RotateArray
RotateArray obj = new RotateArray();
// input 1
int arr[] = {7, 9, 8, 0, 5, 1, 6, 4};
int s = arr.length;
int r = 2;
System.out.println("The input array: ");
for(int k = 0; k < s; k++)
{
System.out.print(arr[k] + " ");
}
System.out.println();
System.out.println("The new array after rotating " + r + " times, we get");
obj.leftRotate(arr, r, s);
for(int k = 0; k < s; k++)
{
System.out.print(arr[k] + " ");
}
System.out.println("\n");
// input 2
int arr1[] = {6, 8, 7, 9, 0, 5, 1, 3, 2, 4};
s = arr1.length;
r = 5;
System.out.println("The input array: ");
for(int k = 0; k < s; k++)
{
System.out.print(arr1[k] + " ");
}
System.out.println();
System.out.println("The new array after rotating " + r + " times, we get");
obj.leftRotate(arr1, r, s);
for(int k = 0; k < s; k++)
{
System.out.print(arr1[k] + " ");
}
}
}
<p><strong>Output:</strong></p>
<div class="codeblock3"><pre>
The input array: 
7 9 8 0 5 1 6 4 
The new array after rotating 2 times, we get
8 0 5 1 6 4 7 9 

The input array: 
6 8 7 9 0 5 1 3 2 4 
The new array after rotating 5 times, we get
5 1 3 2 4 6 8 7 9 0
</pre></div>
<h2 class="h2">Approach: Using Iteration</h2>
<p>Using loops also, one can achieve the same result. The following program shows the same.</p>
<p><strong>FileName:</strong> RotateArray1.java</p>
<div class="codeblock"><textarea name="code" class="java" placeholder="disabled" disabled="disabled" title="form boxes are disabled in cached version of the site" value="Disabled" style="opacity:0.3;" >
public class RotateArray1
{
// It rotates the array elements by r.
public void leftRotate(int input[], int r, int s)
{
int j, k;
// the array will remain the same when size of the 
// input array is equal to the number of times the 
// the array is rotated. Also, if we do not rotate the array 
// at all we get the same array
if(r == 0 || r == s)
{
return;
}
// If the number of elements that have to be rotated is
// greater than the array size
if(r > s)
{
r = r % s;
}
j = r;
k = s - r;
while (j != k)
{
if(j < k) // if A is of short in size
{
swap(input, r - j, r + k - j, j);
k = k - j;
}
else // If B is short in size
{
swap(input, r - j, r, k);
j = j - k;
}
}
// In the end, do the block swap A and B
swap(input , r - j, r, j);
}
// The method swaps the r elements
// beginning at the index f with r elements
// starting at the index g
public void swap(int input[], int f, int g, int r)
{
int k;
int temp;
for(k = 0; k < r; k++)
{
temp = input[f + k];
input[f + k] = input[g + k];
input[g + k] = temp;
}
}
// main method
public static void main (String[] argvs)
{
// creating an object of the class RotateArray1
RotateArray1 obj = new RotateArray1();
// input 1
int arr[] = {7, 9, 8, 0, 5, 1, 6, 4};
int s = arr.length;
int r = 2;
System.out.println("The input array: ");
for(int k = 0; k < s; k++)
{
System.out.print(arr[k] + " ");
}
System.out.println();
System.out.println("The new array after rotating " + r + " times, we get");
obj.leftRotate(arr, r, s);
for(int k = 0; k < s; k++)
{
System.out.print(arr[k] + " ");
}
System.out.println("\n");
// input 2
int arr1[] = {6, 8, 7, 9, 0, 5, 1, 3, 2, 4};
s = arr1.length;
r = 5;
System.out.println("The input array: ");
for(int k = 0; k < s; k++)
{
System.out.print(arr1[k] + " ");
}
System.out.println();
System.out.println("The new array after rotating " + r + " times, we get");
obj.leftRotate(arr1, r, s);
for(int k = 0; k < s; k++)
{
System.out.print(arr1[k] + " ");
}
}
}

Output:

The input array: 
7 9 8 0 5 1 6 4 
The new array after rotating 2 times, we get
8 0 5 1 6 4 7 9 

The input array: 
6 8 7 9 0 5 1 3 2 4 
The new array after rotating 5 times, we get
5 1 3 2 4 6 8 7 9 0

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