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Rearrange array to maximize count of local minima
  • Last Updated : 20 Nov, 2020

Given an array arr[] of size N, the task is to rearrange the array elements such that the count of local minima in the array is maximum.

Note: An element arr[x] is said to be a local minimum if it is less than or equal to both its adjacent elements. The first and last array elements can’t be considered as local minima.

Examples:

Input: arr[]= {1, 2, 3, 4, 5}
Output: 3 1 4 2 5 
Explanation: 
Rearranging array elements to {3, 1, 4, 2, 5}. The count of local minima in the array is 2, i.e. {arr[1], arr[3]}, which is the maximum possible count of local minima that can be obtained from the array. Therefore, the required output is 3 1 4 2 5.

Input: arr[]= {1, 2, 3, 4, 5, 6}
Output: 4 1 5 2 6 3



Approach: The idea is to use sorting algorithms and two pointer technique to solve this problem. Follow the steps below to solve this problem:

  • Sort the array in ascending order.
  • Initialize two variables, say left = 0 and right = N / 2 to store the index of the left and right pointers respectively.
  • Traverse the array and in each traversal, first print the value of arr[right] and then print the value of arr[left] and increment the value of left and right by 1.

Below is the implementation for the above approach:

C++

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// C++ program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to rearrange array elements to
// maximize count of local minima in the array
void rearrangeArrMaxcntMinima(int arr[], int N)
{
    // Sort the array in
    // ascending order
    sort(arr, arr + N);
 
    // Stores index of
    // left pointer
    int left = 0;
 
    // Stores index of
    // right pointer
    int right = N / 2;
 
    // Traverse the array elements
    while (left < N / 2 || right < N) {
 
        // if right is less than N
        if (right < N) {
 
            // Print array element
            cout << arr[right] << " ";
 
            // Update right
            right++;
        }
 
        if (left < N / 2) {
 
            // Print array element
            cout << arr[left] << " ";
 
            // Update left
            left++;
        }
    }
}
 
// Driver Code
int main()
{
 
    int arr[] = { 1, 2, 3, 4, 5 };
 
    int N = sizeof(arr) / sizeof(arr[0]);
 
    rearrangeArrMaxcntMinima(arr, N);
 
    return 0;
}

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Java

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// Java program to implement
// the above approach
import java.util.*;
 
class GFG{
  
// Function to rearrange array elements to
// maximize count of local minima in the array
static void rearrangeArrMaxcntMinima(int arr[],
                                     int N)
{
     
    // Sort the array in
    // ascending order
    Arrays.sort(arr);
  
    // Stores index of
    // left pointer
    int left = 0;
  
    // Stores index of
    // right pointer
    int right = N / 2;
  
    // Traverse the array elements
    while (left < N / 2 || right < N)
    {
         
        // If right is less than N
        if (right < N)
        {
             
            // Print array element
            System.out.print(arr[right] + " ");
  
            // Update right
            right++;
        }
  
        if (left < N / 2)
        {
             
            // Print array element
            System.out.print(arr[left] + " ");
  
            // Update left
            left++;
        }
    }
}
  
// Driver Code
public static void main(String[] args)
{
    int arr[] = { 1, 2, 3, 4, 5 };
  
    int N = arr.length;
  
    rearrangeArrMaxcntMinima(arr, N);
}
}
 
// This code is contributed by code_hunt

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Python3

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# Python3 program to implement
# the above approach
 
# Function to rearrange array
# elements to maximize count of
# local minima in the array
def rearrangeArrMaxcntMinima(arr, N):
 
    # Sort the array in
    # ascending order
    arr.sort()
 
    # Stores index of
    # left pointer
    left = 0
 
    # Stores index of
    # right pointer
    right = N // 2
 
    # Traverse the array elements
    while (left < N // 2 or
           right < N):
 
        # if right is less
        # than N
        if (right < N):
 
            # Print array element
            print(arr[right],
                  end = " ")
 
            # Update right
            right += 1
 
        if (left < N // 2):
 
            # Print array element
            print(arr[left],
                  end = " ")
 
            # Update left
            left += 1
 
# Driver Code
if __name__ == "__main__":
   
    arr = [1, 2, 3, 4, 5]
    N = len(arr)
    rearrangeArrMaxcntMinima(arr, N)
 
# This code is contributed by Chitranayal

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C#

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// C# program to implement
// the above approach
using System;
 
class GFG{
  
// Function to rearrange array elements to
// maximize count of local minima in the array
static void rearrangeArrMaxcntMinima(int []arr,
                                     int N)
{
   
    // Sort the array in
    // ascending order
    Array.Sort(arr);
  
    // Stores index of
    // left pointer
    int left = 0;
  
    // Stores index of
    // right pointer
    int right = N / 2;
  
    // Traverse the array elements
    while (left < N / 2 || right < N)
    {
       
        // If right is less than N
        if (right < N)
        {
             
            // Print array element
            Console.Write(arr[right] + " ");
  
            // Update right
            right++;
        }
        if (left < N / 2)
        {
             
            // Print array element
            Console.Write(arr[left] + " ");
  
            // Update left
            left++;
        }
    }
}
  
// Driver Code
public static void Main(String[] args)
{
    int []arr = { 1, 2, 3, 4, 5 };
    int N = arr.Length;
  
    rearrangeArrMaxcntMinima(arr, N);
}
}
 
// This code is contributed by Rajput-Ji

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Output: 

3 1 4 2 5









 

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

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