Open in App
Not now

# C++ Program to Find Mth element after K Right Rotations of an Array

• Last Updated : 10 Mar, 2022

Given non-negative integers K, M, and an array arr[ ] consisting of N elements, the task is to find the Mth element of the array after K right rotations.

Examples:

Input: arr[] = {3, 4, 5, 23}, K = 2, M = 1
Output:
Explanation:
The array after first right rotation a1[ ] = {23, 3, 4, 5}
The array after second right rotation a2[ ] = {5, 23, 3, 4}
1st element after 2 right rotations is 5.
Input: arr[] = {1, 2, 3, 4, 5}, K = 3, M = 2
Output:
Explanation:
The array after 3 right rotations has 4 at its second position.

Naive Approach:
The simplest approach to solve the problem is to Perform Right Rotation operation K times and then find the Mth element of the final array.
Time Complexity: O(N * K)
Auxiliary Space: O(N)
Efficient Approach:
To optimize the problem, the following observations need to be made:

• If the array is rotated N times it returns the initial array again.

For example, a[ ] = {1, 2, 3, 4, 5}, K=5
Modified array after 5 right rotation a5[ ] = {1, 2, 3, 4, 5}.

• Therefore, the elements in the array after Kth rotation is the same as the element at index K%N in the original array.
• If K >= M, the Mth element of the array after K right rotations is

{ (N-K) + (M-1) } th element in the original array.

• If K < M, the Mth element of the array after K right rotations is:

(M – K – 1) th  element in the original array.

Below is the implementation of the above approach:

## C++

 `// C++ program to implement` `// the above approach` `#include` `using` `namespace` `std;`   `// Function to return Mth element of` `// array after k right rotations` `int` `getFirstElement(``int` `a[], ``int` `N,` `                    ``int` `K, ``int` `M)` `{` `    ``// The array comes to original state` `    ``// after N rotations` `    ``K %= N;` `    ``int` `index;`   `    ``// If K is greater or equal to M` `    ``if` `(K >= M)`   `        ``// Mth element after k right` `        ``// rotations is (N-K)+(M-1) th` `        ``// element of the array` `        ``index = (N - K) + (M - 1);`   `    ``// Otherwise` `    ``else`   `        ``// (M - K - 1) th element` `        ``// of the array` `        ``index = (M - K - 1);`   `    ``int` `result = a[index];`   `    ``// Return the result` `    ``return` `result;` `}`   `// Driver Code ` `int` `main() ` `{ ` `    ``int` `a[] = { 1, 2, 3, 4, 5 }; ` `  `  `    ``int` `N = ``sizeof``(a) / ``sizeof``(a[0]); ` `  `  `    ``int` `K = 3, M = 2; ` `  `  `    ``cout << getFirstElement(a, N, K, M); ` `  `  `    ``return` `0; ` `} `

Output:

`4`

Time complexity: O(1)
Auxiliary Space: O(1)

Please refer complete article on Mth element after K Right Rotations of an Array for more details!

My Personal Notes arrow_drop_up
Related Articles