Minimum replacements required to have at most K distinct elements in the array

Last Updated : 15 Feb, 2021

Given an array arr[] consisting of N positive integers and an integer K, the task is to find the minimum number of array elements required to be replaced by the other array elements such that the array contains at most K distinct elements.

Input: arr[] = { 1, 1, 2, 2, 5 }, K = 2
Output: 1
Explanation:
Replacing arr[4] with arr[0] modifies arr[] to { 1, 1, 2, 2, 1 }
Distinct array elements of the array arr[] are { 1, 2 }
Therefore, the required output is 1.

Input: arr[] = { 5, 1, 3, 2, 4, 1, 1, 2, 3, 4 }, K = 3
Output: 3
Explanation:
Replacing arr[0] with arr[1] modifies arr[] to { 1, 1, 3, 2, 4, 1, 1, 2, 3, 4 }
Replacing arr[2] with arr[0] modifies arr[] to { 1, 1, 1, 2, 4, 1, 1, 2, 3, 4 }
Replacing arr[8] with arr[0] modifies arr[] to { 1, 1, 1, 2, 4, 1, 1, 2, 1, 4 }
Distinct array elements of the array arr[] are { 1, 2, 4 }
Therefore, the required output is 3.

Approach:The problem can be solved using Greedy technique. The idea is to replace the smaller frequency array elements with the higher frequency array elements. Follow the steps below to solve the problem:

Initialize a map, say mp, to store the frequency of each distinct array element.
Traverse the array and store the frequency of each distinct element of the array into the Map.
Traverse the map using the key value of the map as i and insert the value of mp[i] into an array, say Freq[].
Sort the array Freq[] in descending order.
Initialize a variable, say cntMin, to store the minimum count of array elements required to be replaced such that the count of distinct elements in the array is at most K.
Initialize a variable, say len, to store the size of the array Freq[].
Iterate over the range [K, len] using variable i. For every i^th value, update cntMin += Freq[i].
Finally, print the value of cntMin.

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 find minimum count of array
// elements required to be replaced such that
// count of distinct elements is at most K
int min_elements(int arr[], int N, int K)
{
 
    // Store the frequency of each
    // distinct element of the array
    map<int, int> mp;
 
    // Traverse the array
    for (int i = 0; i < N; i++) {
 
        // Update frequency
        // of arr[i]
        mp[arr[i]]++;
    }
 
    // Store frequency of each distinct
    // element of the array
    vector<int> Freq;
 
    // Traverse the map
    for (auto it : mp) {
 
        // Stores key of the map
        int i = it.first;
 
        // Insert mp[i] into Freq[]
        Freq.push_back(mp[i]);
    }
 
    // Sort Freq[] in descending order
    sort(Freq.rbegin(), Freq.rend());
 
    // Stores size of Freq[]
    int len = Freq.size();
 
    // If len is less than
    // or equal to K
    if (len <= K) {
 
        return 0;
    }
 
    // Stores minimum count of array elements
    // required to be replaced such that
    // count of distinct elements is at most K
    int cntMin = 0;
 
    // Iterate over the range [K, len]
    for (int i = K; i < len; i++) {
 
        // Update cntMin
        cntMin += Freq[i];
    }
 
    return cntMin;
}
 
// Driver Code
int main()
{
    int arr[] = { 5, 1, 3, 2, 4, 1, 1, 2, 3, 4 };
 
    int N = sizeof(arr) / sizeof(arr[0]);
 
    int K = 3;
    cout << min_elements(arr, N, K);
    return 0;
}

Java

// Java program to implement
// the above approach
import java.util.*;
class GFG
{
 
  // Function to find minimum count of array
  // elements required to be replaced such that
  // count of distinct elements is at most K
  static int min_elements(int arr[], int N, int K)
  {
 
    // Store the frequency of each
    // distinct element of the array
    HashMap<Integer,Integer> mp = new HashMap<Integer,Integer>();
 
    // Traverse the array
    for (int i = 0; i < N; i++)
    {
 
      // Update frequency
      // of arr[i]
      if(mp.containsKey(arr[i]))
      {
        mp.put(arr[i], mp.get(arr[i])+1);
      }
      else
      {
        mp.put(arr[i], 1);
      }
    }
 
    // Store frequency of each distinct
    // element of the array
    Vector<Integer> Freq = new Vector<Integer>();
 
    // Traverse the map
    for (Map.Entry<Integer,Integer> it : mp.entrySet()) 
    {
 
      // Stores key of the map
      int i = it.getKey();
 
      // Insert mp[i] into Freq[]
      Freq.add(mp.get(i));
    }
 
    // Sort Freq[] in descending order
    Collections.sort(Freq,Collections.reverseOrder());
 
    // Stores size of Freq[]
    int len = Freq.size();
 
    // If len is less than
    // or equal to K
    if (len <= K) 
    {
      return 0;
    }
 
    // Stores minimum count of array elements
    // required to be replaced such that
    // count of distinct elements is at most K
    int cntMin = 0;
 
    // Iterate over the range [K, len]
    for (int i = K; i < len; i++)
    {
 
      // Update cntMin
      cntMin += Freq.get(i);
    }
    return cntMin;
  }
 
  // Driver Code
  public static void main(String[] args)
  {
    int arr[] = { 5, 1, 3, 2, 4, 1, 1, 2, 3, 4 };
    int N = arr.length;
    int K = 3;
    System.out.print(min_elements(arr, N, K));
  }
}
 
// This code is contributed by shikhasingrajput

Python3

# Python3 program to implement
# the above approach
 
# Function to find minimum count of array
# elements required to be replaced such that
# count of distinct elements is at most K
def min_elements(arr, N, K) :
  
    # Store the frequency of each
    # distinct element of the array
    mp = {}
  
    # Traverse the array
    for i in range(N) :
  
        # Update frequency
        # of arr[i]
        if arr[i] in mp :
            mp[arr[i]] += 1
        else :
            mp[arr[i]] = 1
  
    # Store frequency of each distinct
    # element of the array
    Freq = []
  
    # Traverse the map
    for it in mp :
  
        # Stores key of the map
        i = it
  
        # Insert mp[i] into Freq[]
        Freq.append(mp[i])
  
    # Sort Freq[] in descending order
    Freq.sort()
    Freq.reverse()
  
    # Stores size of Freq[]
    Len = len(Freq)
  
    # If len is less than
    # or equal to K
    if (Len <= K) :
        return 0
  
    # Stores minimum count of array elements
    # required to be replaced such that
    # count of distinct elements is at most K
    cntMin = 0
  
    # Iterate over the range [K, len]
    for i in range(K, Len) :
  
        # Update cntMin
        cntMin += Freq[i]
  
    return cntMin
     
  # Driver code
arr = [ 5, 1, 3, 2, 4, 1, 1, 2, 3, 4 ]
N = len(arr)
K = 3;
print(min_elements(arr, N, K))
 
# This code is contributed by divyesh072019.

C#

// C# program to implement
// the above approach
using System;
using System.Collections.Generic;
 
class GFG{
 
// Function to find minimum count of array
// elements required to be replaced such that
// count of distinct elements is at most K
static int min_elements(int []arr, int N, int K)
{
     
    // Store the frequency of each
    // distinct element of the array
    Dictionary<int,
               int> mp = new Dictionary<int,
                                        int>();
     
    // Traverse the array
    for(int i = 0; i < N; i++)
    {
         
        // Update frequency
        // of arr[i]
        if (mp.ContainsKey(arr[i]))
        {
            mp[arr[i]] = mp[arr[i]] + 1;
        }
        else
        {
            mp.Add(arr[i], 1);
        }
    }
     
    // Store frequency of each distinct
    // element of the array
    List<int> Freq = new List<int>();
     
    // Traverse the map
    foreach (KeyValuePair<int, int> it in mp) 
    {
         
        // Stores key of the map
        int i = it.Key;
         
        // Insert mp[i] into Freq[]
        Freq.Add(mp[i]);
    }
     
    // Sort Freq[] in descending order
    Freq.Sort();
    Freq.Reverse();
     
    // Stores size of Freq[]
    int len = Freq.Count;
     
    // If len is less than
    // or equal to K
    if (len <= K) 
    {
        return 0;
    }
     
    // Stores minimum count of array elements
    // required to be replaced such that
    // count of distinct elements is at most K
    int cntMin = 0;
     
    // Iterate over the range [K, len]
    for(int i = K; i < len; i++)
    {
         
        // Update cntMin
        cntMin += Freq[i];
    }
    return cntMin;
}
 
// Driver Code
public static void Main(String[] args)
{
    int []arr = { 5, 1, 3, 2, 4, 1, 1, 2, 3, 4 };
    int N = arr.Length;
    int K = 3;
     
    Console.Write(min_elements(arr, N, K));
}
}
 
// This code is contributed by gauravrajput1

Output:

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

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