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Maximize count of 1s in an array by repeated division of array elements by 2 at most K times
  • Difficulty Level : Hard
  • Last Updated : 19 Feb, 2021

Given an array arr[] of size N and an integer K, the task to find the maximum number of array elements that can be reduced to 1 by repeatedly dividing any element by 2 at most K times. 
Note: For odd array elements, take its ceil value of division.

Examples:

Input: arr[] = {5, 8, 4, 7}, K = 5
Output: 2
Explanation:
5 needs 3 operations(5→3→2→1).
8 needs 3 operations(8→4→2→1). 
4 needs 2 operations(4→2→1).     
7 needs 3 operations(7→4→2→1)
Therefore, in 5 operations, the maximum number of array elements that can be reduced to 1 are 2, either (4, 5), (4, 8) or (4, 7).

Input: arr[] = {5, 8, 5, 7}, K = 5
Output: 1

Approach: To maximize the number of elements, the idea is to sort the array in ascending order and start reducing the elements from the first index and decrement K by the number of operations required to reduce the ith element. Follow the steps below to solve the problem:

  • Initialize a variable, say cnt, to store the required number of elements.
  • Sort the array arr[] in increasing order.
  • Traverse the array, arr[] using the variable i, and perform the following steps:
    • Store the number of operations required to reduce arr[i] to 1 is opr = ceil(log2(arr[i])).
    • Decrement K by opr.
    • If the value of K becomes less than 0, break out of the loop. Otherwise, increment cnt by 1.
  • After completing the above steps, print the value of cnt as the result.

Below is the implementation of the above approach:



C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to count the maximum number of
// array elements that can be reduced to 1
// by repeatedly dividing array elements by 2
void findMaxNumbers(int arr[], int n, int k)
{
    // Sort the array in ascending order
    sort(arr, arr + n);
 
    // Store the count of array elements
    int cnt = 0;
 
    // Traverse the array
    for (int i = 0; i < n; i++) {
 
        // Store the number of operations
        // required to reduce arr[i] to 1
        int opr = ceil(log2(arr[i]));
 
        // Decrement k by opr
        k -= opr;
 
        // If k becomes less than 0,
        // then break out of the loop
        if (k < 0) {
            break;
        }
 
        // Increment cnt by 1
        cnt++;
    }
 
    // Print the answer
    cout << cnt;
}
 
// Driver Code
int main()
{
    int arr[] = { 5, 8, 4, 7 };
    int N = sizeof(arr) / sizeof(arr[0]);
    int K = 5;
 
    findMaxNumbers(arr, N, K);
 
    return 0;
}

Java




// Java program for the above approach
import java.util.*;
public class GFG
{
 
// Function to count the maximum number of
// array elements that can be reduced to 1
// by repeatedly dividing array elements by 2
static void findMaxNumbers(int arr[], int n, int k)
{
   
    // Sort the array in ascending order
    Arrays.sort(arr);
 
    // Store the count of array elements
    int cnt = 0;
 
    // Traverse the array
    for (int i = 0; i < n; i++)
    {
 
        // Store the number of operations
        // required to reduce arr[i] to 1
        int opr = (int)Math.ceil((Math.log(arr[i]) / Math.log(2)));
 
        // Decrement k by opr
        k -= opr;
 
        // If k becomes less than 0,
        // then break out of the loop
        if (k < 0) {
            break;
        }
 
        // Increment cnt by 1
        cnt++;
    }
 
    // Print the answer
    System.out.println(cnt);
}
 
// Driver Code
public static void main(String args[])
{
    int arr[] = { 5, 8, 4, 7 };
    int N = arr.length;
    int K = 5;
    findMaxNumbers(arr, N, K);
}
}
 
// This code is contributed by jana_sayantan.

Python3




# Python3 program to implement
# the above approach
import math
 
# Function to count the maximum number of
# array elements that can be reduced to 1
# by repeatedly dividing array elements by 2
def findMaxNumbers(arr, n, k) :
     
    # Sort the array in ascending order
    arr.sort()
 
    # Store the count of array elements
    cnt = 0
 
    # Traverse the array
    for i in range(n):
 
        # Store the number of operations
        # required to reduce arr[i] to 1
        opr = math.ceil(math.log2(arr[i]))
 
        # Decrement k by opr
        k -= opr
 
        # If k becomes less than 0,
        # then break out of the loop
        if (k < 0) :
            break
         
        # Increment cnt by 1
        cnt += 1
     
    # Prthe answer
    print(cnt)
 
# Driver Code
arr = [ 5, 8, 4, 7 ]
N = len(arr)
K = 5
 
findMaxNumbers(arr, N, K)
 
# This code is contributed by splevel62.

C#




// C# program for the above approach
using System;
public class GFG
{
   
// Function to count the maximum number of
// array elements that can be reduced to 1
// by repeatedly dividing array elements by 2
static void findMaxNumbers(int[] arr, int n, int k)
{
   
    // Sort the array in ascending order
    Array.Sort(arr);
 
    // Store the count of array elements
    int cnt = 0;
 
    // Traverse the array
    for (int i = 0; i < n; i++)
    {
 
        // Store the number of operations
        // required to reduce arr[i] to 1
        int opr = (int)Math.Ceiling((Math.Log(arr[i]) / Math.Log(2)));
 
        // Decrement k by opr
        k -= opr;
 
        // If k becomes less than 0,
        // then break out of the loop
        if (k < 0) {
            break;
        }
 
        // Increment cnt by 1
        cnt++;
    }
 
    // Print the answer
    Console.Write(cnt);
}
 
// Driver Code
public static void Main(String[] args)
{
    int[] arr = { 5, 8, 4, 7 };
    int N = arr.Length;
    int K = 5;
    findMaxNumbers(arr, N, K);
}
}
 
// This code is contributed by susmitakundugoaldanga.

 
 

Output: 
2

 

 

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

 

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