Hello Everyone,
Question: 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]
The first 3 methods to rotate an array by d elements has been discussed in this, ( Program for array rotation ) 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 :
- C++
// C++ program for reversal algorithm
// of array rotation
#include <bits/stdc++.h>
using namespace std;
/*Function to reverse arr[] from index start to end*/
void reverseArray( int arr[], int start, int end)
{
while (start < end) {
int temp = arr[start];
arr[start] = arr[end];
arr[end] = temp;
start++;
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);
}
// Function to print an array
void printArray( int arr[], int size)
{
for ( int i = 0; i < size; i++)
cout << arr[i] << " " ;
}
/* 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;
// Function calling
leftRotate(arr, d, n);
printArray(arr, n);
return 0;
}
Output :
3 4 5 6 7 1 2
Time Complexity : O(n)
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above