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Javascript Program for Reversal algorithm for array rotation

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  • Last Updated : 28 May, 2022
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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

 

The first 3 methods to rotate an array by d elements has been discussed in this post. 
Method 4 (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 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 implementation of the above approach : 
 

Javascript




<script>
      // JavaScript program for reversal algorithm
      // of array rotation
 
      /*Function to reverse arr[] from index start to end*/
      function reverseArray(arr, start, end) {
        while (start < end) {
          var temp = arr[start];
          arr[start] = arr[end];
          arr[end] = temp;
          start++;
          end--;
        }
      }
 
      /* 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 print an array
      function printArray(arr, size)
      {
        for (var i = 0; i < size; i++) document.write(arr[i] + " ");
      }
 
      /* Driver program to test above functions */
 
      var arr = [1, 2, 3, 4, 5, 6, 7];
      var n = arr.length;
      var d = 2;
 
      // Function calling
      leftRotate(arr, d, n);
      printArray(arr, n);
       
      // This code is contributed by rdtank.
    </script>


Output : 

3 4 5 6 7 1 2

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!


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