Minimum delete operations to make all elements of array same

Given an array of n elements such that elements may repeat. We can delete any number of elements from the array. The task is to find a minimum number of elements to be deleted from the array to make it equal.
Examples:

Input: arr[] = {4, 3, 4, 4, 2, 4}
Output: 2
After deleting 2 and 3 from array, array becomes 
arr[] = {4, 4, 4, 4} 

Input: arr[] = {1, 2, 3, 4, 5}
Output: 4
We can delete any four elements from array.

In this problem, we need to minimize the delete operations. The approach is simple, we count the frequency of each element in an array, then find the frequency of the most frequent elements in count array. Let this frequency be max_freq. To get the minimum number of elements to be deleted from the array calculate n – max_freq where n is a number of elements in the given array.

C++

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// C++ program to find minimum
// number of deletes required
// to make all elements same.
#include <bits/stdc++.h>
using namespace std;
 
// Function to get minimum number of elements to be deleted
// from array to make array elements equal
int minDelete(int arr[], int n)
{
    // Create an hash map and store frequencies of all
    // array elements in it using element as key and
    // frequency as value
    unordered_map<int, int> freq;
    for (int i = 0; i < n; i++)
        freq[arr[i]]++;
 
    // Find maximum frequency among all frequencies.
    int max_freq = INT_MIN;
    for (auto itr = freq.begin(); itr != freq.end(); itr++)
        max_freq = max(max_freq, itr->second);
 
    // To minimize delete operations, we remove all
    // elements but the most frequent element.
    return n - max_freq;
}
 
// Driver program to run the case
int main()
{
    int arr[] = { 4, 3, 4, 4, 2, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << minDelete(arr, n);
    return 0;
}

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Java

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// Java program to find minimum number
// of deletes required to make all
// elements same.
import java.util.*;
 
class GFG{
 
// Function to get minimum number of
// elements to be deleted from array
// to make array elements equal
static int minDelete(int arr[], int n)
{
     
    // Create an hash map and store
    // frequencies of all array elements
    // in it using element as key and
    // frequency as value
    HashMap<Integer, Integer> freq = new HashMap<>();
    for(int i = 0; i < n; i++)
        freq.put(arr[i], freq.getOrDefault(arr[i], 0) + 1);
 
    // Find maximum frequency among all frequencies.
    int max_freq = Integer.MIN_VALUE;
    for(Map.Entry<Integer,
                  Integer> entry : freq.entrySet())
        max_freq = Math.max(max_freq,
                            entry.getValue());
 
    // To minimize delete operations,
    // we remove all elements but the
    // most frequent element.
    return n - max_freq ;
}
 
// Driver code
public static void main(String[] args)
{
    int arr[] = { 4, 3, 4, 4, 2, 4 };
    int n = arr.length;
     
    System.out.print(minDelete(arr, n));
}
}
 
// This code is contributed by amal kumar choubey and corrected by Leela Kotte

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Python3

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# Python3 program to find minimum
# number of deletes required to
# make all elements same.
 
# Function to get minimum number
# of elements to be deleted from
# array to make array elements equal
def minDelete(arr, n):
     
    # Create an dictionary and store
    # frequencies of all array
    # elements in it using
    # element as key and
    # frequency as value
    freq = {}
    for i in range(n):
        if arr[i] in freq:
            freq[arr[i]] += 1
        else:
            freq[arr[i]] = 1;
         
 
    # Find maximum frequency
    # among all frequencies.
    max_freq = 0;
    for i, j in freq.items():
        max_freq = max(max_freq, j);
 
    # To minimize delete operations,
    # we remove all elements but the
    # most frequent element.
    return n - max_freq;
     
# Driver code
arr = [ 4, 3, 4, 4, 2, 4 ];
n = len(arr)
 
print(minDelete(arr, n));
 
# This code is contributed by grand_master

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C#

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// C# program to find minimum number
// of deletes required to make all
// elements same.
using System;
using System.Collections.Generic;
class GFG {
 
    // Function to get minimum number of
    // elements to be deleted from array
    // to make array elements equal
    static int minDelete(int[] arr, int n)
    {
 
        // Create an hash map and store
        // frequencies of all array elements
        // in it using element as key and
        // frequency as value
        Dictionary<int, int> freq
            = new Dictionary<int, int>();
        for (int i = 0; i < n; i++)
            if (freq.ContainsKey(arr[i]))
            {
                freq[arr[i]] = freq[arr[i]] + 1;
            }
            else
            {
                freq.Add(arr[i], 1);
            }
 
        // Find maximum frequency among all frequencies.
        int max_freq = int.MinValue;
        foreach(KeyValuePair<int, int> entry in freq)
            max_freq = Math.Max(max_freq, entry.Value);
 
        // To minimize delete operations,
        // we remove all elements but the
        // most frequent element.
        return n - max_freq + 1;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        int[] arr = {4, 3, 4, 4, 2, 4};
        int n = arr.Length;
          Console.Write(minDelete(arr, n));
    }
}
 
// This code is contributed by Amit Katiyar

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Output:

2

Time complexity: O(n)

Note: Here we can optimize the extra space to count the frequency of each element to O(1) but for this, we have to modify our original array. See this article.

This article is contributed by Shashank Mishra ( Gullu ). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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