C 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 array
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 C implementation of the above approach :
C++
// C/C++ program for reversal algorithm of array rotation #include <stdio.h> /*Utility function to print an array */ void printArray( int arr[], int size); /* Utility function to reverse arr[] from start to end */ void reverseArray( int arr[], int start, int end); /* Function to left rotate arr[] of size n by d */ void leftRotate( int arr[], int d, int 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); } /*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]); } /*Function to reverse arr[] from index start to end*/ void reverseArray( int arr[], int start, int end) { 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 }; int n = sizeof (arr) / sizeof (arr[0]); int d = 2; leftRotate(arr, d, n); printArray(arr, n); return 0; } |
Output
3 4 5 6 7 1 2
Time Complexity : O(n)
Auxiliary Space : O(1)
Please refer complete article on Reversal algorithm for array rotation for more details!