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Count of possible rotations of given Array to remove largest element from first half

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  • Difficulty Level : Medium
  • Last Updated : 28 Mar, 2022
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Given an array arr[ ] with even length N, the task is to find the number of cyclic shifts (rotations) possible for this array, such that the first half of array does not contain the maximum element.

Examples:

Input: N = 6, arr[ ] = { 3, 3, 5, 3, 3, 3 }
Output: 3
Explanation: The maximum element here is 5 at index 2. This 5 can be shifted to second half of the array using any of the below three valid right shifts:

  • shift by 1: (3, 3, 3, 5, 3, 3)
  • shift by 2: (3, 3, 3, 3, 5, 3)
  • shift by 3: (3, 3, 3, 3, 3, 5)

Input: N = 6, arr[ ] = {8, 8, 9, 8, 8, 9}
Output: 0
Explanation: Any rotation in the article wont help in removing the maximum element from 1st half, as there are two occurrences of 9 at index 2 and 5. So any rotation will keep atleast one 9 at index 0, 1 or 2.

 

Naive Approach: The most basic approach to solve this problem, is based on below idea:

Find all rotations of given array. In the end, just return the count of such rotations which do not have the maximum element in first half.

Time Complexity: O(N3) where N^2 is for rotations and N is for finding maximum in each rotation.
Auxiliary Space: O(1)

Efficient Approach: The idea to solve the problem is by traversing the array and some basic concepts of maths. 

The idea is to find distance between maximum elements and simply check if the range is greater than half of array size. If yes, rotation is possible, or else rotation is not possible.

For case when rotation is possible, we can find the [range – (n/2)] as the valid number of rotations.

Follow the steps to solve the problem:

  • Firstly, find the maximum element
  • Then find the maximal ranges between pairs of maximum elements.
  • A range with length m adds max(m-frac(N/2)+1, 0) to the answer.

Below is the implementation of the above approach: 

C++




// C++ program for the above approach.
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the number of cyclic shifts
int find(int arr[], int N)
{
    int maxele = *max_element(arr, arr + N);
    int left = -1;
    int right = -1;
 
    // Placing left pointer
    // On its correct position
    for (int i = 0; i < N; i++) {
        if (arr[i] == maxele) {
            left = i;
            break;
        }
    }
 
    // Placing right pointer
    // On its correct position
    for (int i = N - 1; i >= 0; i--) {
        if (arr[i] == maxele) {
            right = i;
            break;
        }
    }
    int ans = (N / 2) - (right - left);
    if (ans <= 0) {
        return 0;
    }
    else {
        return ans;
    }
}
 
// Driver Code
int main()
{
    int arr[] = { 3, 3, 5, 3, 3, 3 };
    int N = sizeof(arr) / sizeof(arr[0]);
 
    // Function call
    cout << find(arr, N);
    return 0;
}

Java




// Java program for the above approach.
import java.io.*;
class GFG {
 
  // Function to find the number of cyclic shifts
  static int find(int arr[], int N)
  {
    int maxele = Integer.MIN_VALUE;
    for(int i  = 0; i < N; i++){
      maxele = Math.max(arr[i], maxele);
    }
    int left = -1;
    int right = -1;
 
    // Placing left pointer
    // On its correct position
    for (int i = 0; i < N; i++) {
      if (arr[i] == maxele) {
        left = i;
        break;
      }
    }
 
    // Placing right pointer
    // On its correct position
    for (int i = N - 1; i >= 0; i--) {
      if (arr[i] == maxele) {
        right = i;
        break;
      }
    }
    int ans = (N / 2) - (right - left);
    if (ans <= 0) {
      return 0;
    }
    else {
      return ans;
    }
  }
 
  // Driver Code
  public static void main (String[] args) {
    int arr[] = { 3, 3, 5, 3, 3, 3 };
    int N = arr.length;
 
    // Function call
    System.out.print(find(arr, N));
  }
}
 
// This code is contributed by hrithikgarg03188.

Python3




# Python code for the above approach
 
# Function to find the number of cyclic shifts
def find(arr, N):
    maxele = max(arr)
    left = -1
    right = -1
 
    # Placing left pointer
    # On its correct position
    for i in range(N):
        if (arr[i] == maxele):
            left = i
            break
 
    # Placing right pointer
    # On its correct position
    for i in range(N - 1, -1, -1):
        if (arr[i] == maxele):
            right = i
            break
    ans = (N // 2) - (right - left)
    if (ans <= 0):
        return 0
    else:
        return ans
 
# Driver Code
arr = [3, 3, 5, 3, 3, 3]
N = len(arr)
 
# Function call
print(find(arr, N))
 
# This code is contributed by shinjanpatra

C#




// C# program for the above approach.
using System;
class GFG {
 
    // Function to find the number of cyclic shifts
    static int find(int[] arr, int N)
    {
        int maxele = Int32.MinValue;
        for (int i = 0; i < N; i++) {
            maxele = Math.Max(arr[i], maxele);
        }
        int left = -1;
        int right = -1;
 
        // Placing left pointer
        // On its correct position
        for (int i = 0; i < N; i++) {
            if (arr[i] == maxele) {
                left = i;
                break;
            }
        }
 
        // Placing right pointer
        // On its correct position
        for (int i = N - 1; i >= 0; i--) {
            if (arr[i] == maxele) {
                right = i;
                break;
            }
        }
        int ans = (N / 2) - (right - left);
        if (ans <= 0) {
            return 0;
        }
        else {
            return ans;
        }
    }
 
    // Driver Code
    public static void Main()
    {
        int[] arr = { 3, 3, 5, 3, 3, 3 };
        int N = arr.Length;
 
        // Function call
        Console.Write(find(arr, N));
    }
}
 
// This code is contributed by Samim Hossain Mondal.

Javascript




<script>
       // JavaScript code for the above approach
 
       // Function to find the number of cyclic shifts
       function find(arr, N) {
           let maxele = Math.max([...arr]);
           let left = -1;
           let right = -1;
 
           // Placing left pointer
           // On its correct position
           for (let i = 0; i < N; i++) {
               if (arr[i] == maxele) {
                   left = i;
                   break;
               }
           }
 
           // Placing right pointer
           // On its correct position
           for (let i = N - 1; i >= 0; i--) {
               if (arr[i] == maxele) {
                   right = i;
                   break;
               }
           }
           let ans = Math.floor(N / 2) - (right - left);
           if (ans <= 0) {
               return 0;
           }
           else {
               return ans;
           }
       }
 
       // Driver Code
 
       let arr = [3, 3, 5, 3, 3, 3];
       let N = arr.length;
 
       // Function call
       document.write(find(arr, N));
 
     // This code is contributed by Potta Lokesh
 
   </script>

 
 

Output

3

 

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

 


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