PHP 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] 

Rotation of the above array by 2 will make an array

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 the reverse of A.
• Reverse B to get ArBr, where Br is the 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 : 

<?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
?>

3 4 5 6 7 1 2 

Complexity Analysis:

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