C++ Program to Count of rotations required to generate a sorted array

Last Updated : 25 Jan, 2022
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Given an array arr[], the task is to find the number of rotations required to convert the given array to sorted form.
Examples:

Input: arr[] = {4, 5, 1, 2, 3} 
Output: 2 
Explanation: 
Sorted array {1, 2, 3, 4, 5} after 2 anti-clockwise rotations.

Input: arr[] = {2, 1, 2, 2, 2} 
Output: 1 
Explanation: 
Sorted array {1, 2, 2, 2, 2} after 1 anti-clockwise rotations.

Naive Approach:
To solve the problem mentioned above the first observation is if we have n elements in the array then after sorting, the largest element is at (n - 1)th position. After k number of anti-clockwise rotations, the largest element will be at index (k - 1) (kth element from start). Another thing to note here is that, after rotation, the next element of the largest element will always be the smallest element, (unless the largest element is at last index, possible if there was no rotation). 
Hence,

Number of rotations (k) = index of smallest element (k) in the array

Below is the implementation of the above approach:

C++ 
// C++ program to find the
// count of rotations
#include <bits/stdc++.h> 
using namespace std;

// Function to return the count 
// of rotations
int countRotation(int arr[], int n)
{
    for(int i = 1; i < n; i++)
    {
        
       // Find the smallest element
       if (arr[i] < arr[i - 1])
       {
           // Return its index
           return i;
       }
    }
    
    // If array is not
    // rotated at all
    return 0;
}

// Driver Code
int main()
{
    int arr1[] = { 4, 5, 1, 2, 3 };
    int n = sizeof(arr1) / sizeof(int);
    
    cout << countRotation(arr1, n);
}

// This code is contributed by jrishabh99

Output: 
2

 

Time Complexity: O(N) 
Auxiliary Space: O(1)

Efficient Approach: 
To optimize the above approach, we will use Binary Search. We can notice that, after being sorted and rotated, the given array is divided into two halves with non-decreasing elements, which is the only pre-requisite for binary search. Perform a recursive binary search in the array to find the index of the smallest element.

Below is the implementation of the above approach:

C++ 
// C++ program to implement the 
// above approach 
#include <bits/stdc++.h>
using namespace std;

// Function to return the 
// count of rotations 
int countRotation(int arr[], int low,
                            int high) 
{ 
    
    // If array is not rotated 
    if (low > high)
    { 
        return 0; 
    } 

    int mid = low + (high - low) / 2; 

    // Check if current element is 
    // greater than the next 
    // element 
    if (mid < high && arr[mid] > arr[mid + 1])
    { 
        
        // The next element is 
        // the smallest 
        return mid + 1; 
    } 

    // Check if current element is 
    // smaller than it's previous 
    // element 
    if (mid > low && arr[mid] < arr[mid - 1])
    { 
        
        // Current element is 
        // the smallest 
        return mid; 
    } 

    // Check if current element is 
    // greater than lower bound 
    if (arr[mid] > arr[low]) 
    {
        
        // The sequence is increasing 
        // so far 
        // Search for smallest 
        // element on the right 
        // subarray 
        return countRotation(arr, mid + 1, 
                            high); 
    } 

    if (arr[mid] < arr[high])
    { 
        
        // Smallest element lies on the 
        // left subarray 
        return countRotation(arr, low, 
                            mid - 1); 
    } 
    else
    { 
        
        // Search for the smallest 
        // element on both subarrays 
        int rightIndex = countRotation(arr, 
                                    mid + 1, 
                                    high); 
        int leftIndex = countRotation(arr, low, 
                                    mid - 1); 
        if (rightIndex == 0)
        { 
            return leftIndex; 
        } 
        return rightIndex; 
    } 
} 

// Driver code 
int main()
{
    int arr1[] = { 4, 5, 1, 2, 3 }; 
    int N = sizeof(arr1) / sizeof(arr1[0]);
    
    cout << countRotation(arr1, 0, N - 1);
    
    return 0;
}

// This code is contributed by divyeshrabadiya07

Output: 
2

 

Time Complexity: O(N) 
The complexity will be O(logN) for an array without duplicates. But if the array contains duplicates, then it will recursively call the search for both halves. So the worst-case complexity will be O(N).

Auxiliary Space:O(N) 
At worst case, the recursion call stack will have N/2 recursion calls at a time.
 

Please refer complete article on Count of rotations required to generate a sorted array for more details!

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