Saturday, 24 October 2015

Reversal algorithm for array rotation

Write a function rotate(arr[], d, n) that rotates arr[] of size n by d elements.
Array
Rotation of the above array by 2 will make array
ArrayRotation1


Method 4(The Reversal Algorithm)
Please read this for first three methods of array rotation.
Algorithm:
rotate(arr[], d, n)
  reverse(arr[], 1, d) ;
  reverse(arr[], d + 1, n);
  reverse(arr[], l, 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. /* Ar is reverse of A */
Reverse B to get ArBr. /* Br is reverse of B */
Reverse all to get (ArBr) r = BA.
For 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]
Implementation:
/*Utility function to print an array */
void printArray(int arr[], int size);
 
/* Utility function to reverse arr[] from start to end */
void rvereseArray(int arr[], int start, int end);
 
/* Function to left rotate arr[] of size n by d */
void leftRotate(int arr[], int d, int n)
{
  rvereseArray(arr, 0, d-1);
  rvereseArray(arr, d, n-1);
  rvereseArray(arr, 0, n-1);
}
 
/*UTILITY FUNCTIONS*/
/* function to print an array */
void printArray(int arr[], int size)
{
  int i;
  for(i = 0; i < size; i++)
    printf("%d ", arr[i]);
  printf("%\n ");
}
 
/*Function to reverse arr[] from index start to end*/
void rvereseArray(int arr[], int start, int end)
{
  int i;
  int temp;
  while(start < end)
  {
    temp = arr[start];
    arr[start] = arr[end];
    arr[end] = temp;
    start++;
    end--;
  }
}
 
/* Driver program to test above functions */
int main()
{
   int arr[] = {1, 2, 3, 4, 5, 6, 7};
   leftRotate(arr, 2, 7);
   printArray(arr, 7);
   getchar();
   return 0;
}
Time Complexity: O(n)

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