Minimize deletions in Array by deleting all occurrences of any number such that array size is reduced to at least half

• Difficulty Level : Medium
• Last Updated : 14 Jun, 2022

Given an array arr[] of positive integers, the task is to select an element from the array and delete all its occurrences, such that the number of elements selected are minimum and the array size becomes atleast half of its original size.
Note: Size of the given array is always even.

Example:

Input: arr[] = {2, 2, 2, 2, 4, 4, 6, 7, 7, 3, 3, 3}
Output: 2
Explanation: First we select 2 and delete all its occurrences after that arr[] becomes – {4, 4, 6, 7, 7, 3, 3, 3} with size = 8. As the size is still greater than half, we select 3 and delete all its occurrences, after that arr[] becomes – {4, 4, 6, 7, 7} with size = 5.

Input: arr[] = {3, 3, 3, 3, 3}
Output: 1
Explanation : select 3 and remove all its occurrences.

Input: arr[] = {1, 2, 3, 4, 5, 6, 7, 8}
Output: 4
Explanation : Since there is no duplicate elements, hence any 4 elements can be removed to make the array length at least half.

Approach: The task can be easily achieved by removing the elements with maximum frequency, as soon as the array size becomes at least half of the actual size, we return the number of unique elements deleted till now.

Follow the steps to solve the problem:

• Use Hash-map to store frequency of the elements in array present.
• Store the frequencies in a list.
• Sort the list and traverse it from the back.
• Select the largest frequency element and decrement it from the array size and increment the count of unique elements deleted.
• If new array size becomes at-least half of the original array size, return the number of unique elements till now.

Below is the implementation of the above approach:

C++

 `// C++ Program to implement``// the above approach``#include ``using` `namespace` `std;`  `    ``// Function to calculate the minimum``    ``// elements removed``     ``int` `reduceArrSize(``int` `arr[],``int` `n)``    ``{``        ``unordered_map<``int``,``int``> hm;` `        ``// Making frequency map of elements``        ``for` `(``int` `i = 0; i < n; i++) {``            ``hm[arr[i]]++;``        ``}` `        ``// Storing frequencies in a list``        ``vector<``int``> freq;` `        ``for``(``auto` `it = hm.begin(); it != hm.end(); it++)``        ``{``            ``freq.push_back(it->second);``        ``}` `        ``// Sorting the list``        ``sort(freq.begin(), freq.end());` `        ``int` `size = n;``        ``int` `idx = freq.size() - 1;``        ``int` `count = 0;` `        ``// Counting number of elements to be deleted``        ``while` `(size > n/ 2) {``            ``size -= freq[idx--];``            ``count++;``        ``}``        ``return` `count;``    ``}` `    ``// Driver Code``    ``int` `main()``    ``{``        ``int` `arr[] = { 2, 2, 2, 2, 4, 4,``                      ``6, 7, 7, 3, 3, 3 };``        ``int` `n = ``sizeof``(arr)/``sizeof``(arr[0]);``        ``int` `count = reduceArrSize(arr, n);``        ``cout<<(count);``        ``return` `0;``    ``}` `// This code is contributed by Potta Lokesh`

Java

 `// Java program for the above approach``import` `java.util.*;` `class` `GFG {` `    ``// Function to calculate the minimum``    ``// elements removed``    ``public` `static` `int` `reduceArrSize(``int``[] arr)``    ``{``        ``HashMap hm = ``new` `HashMap<>();` `        ``// Making frequency map of elements``        ``for` `(``int` `i = ``0``; i < arr.length; i++) {``            ``hm.put(arr[i], hm.getOrDefault(arr[i], ``0``) + ``1``);``        ``}` `        ``// Storing frequencies in a list``        ``ArrayList freq``            ``= ``new` `ArrayList(hm.values());` `        ``// Sorting the list``        ``Collections.sort(freq);` `        ``int` `size = arr.length;``        ``int` `idx = freq.size() - ``1``;``        ``int` `count = ``0``;` `        ``// Counting number of elements to be deleted``        ``while` `(size > arr.length / ``2``) {``            ``size -= freq[idx--];``            ``count++;``        ``}``        ``return` `count;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int``[] arr = { ``2``, ``2``, ``2``, ``2``, ``4``, ``4``,``                      ``6``, ``7``, ``7``, ``3``, ``3``, ``3` `};``        ``int` `count = reduceArrSize(arr);``        ``System.out.println(count);``    ``}``}`

Python3

 `# python 3 Program to implement``# the above approach` `# Function to calculate the minimum``# elements removed``def` `reduceArrSize(arr,n):``    ``hm ``=` `{}` `    ``# Making frequency map of elements``    ``for` `i ``in` `range``(n):``        ``if` `arr[i] ``in` `hm:``            ``hm[arr[i]] ``+``=` `1``        ``else``:``            ``hm[arr[i]] ``=` `1` `    ``# Storing frequencies in a list``    ``freq ``=` `[]` `    ``for` `key,value ``in` `hm.items():``        ``freq.append(value)` `    ``# Sorting the list``    ``freq.sort()` `    ``size ``=` `n``    ``idx ``=` `len``(freq) ``-` `1``    ``count ``=` `0` `    ``# Counting number of elements to be deleted``    ``while` `(size > n``/``/` `2``):``        ``size ``-``=` `freq[idx]``        ``idx ``-``=` `1``        ``count ``+``=` `1``    ``return` `count` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``    ``arr ``=` `[``2``, ``2``, ``2``, ``2``, ``4``, ``4``,``6``, ``7``, ``7``, ``3``, ``3``, ``3``]``    ``n ``=` `len``(arr)``    ``count ``=` `reduceArrSize(arr, n)``    ``print``(count)``    ` `    ``# This code is contributed by SURENDRA_GANGWAR.`

C#

 `// C# program for the above approach``using` `System;``using` `System.Collections.Generic;` `public` `class` `GFG {` `    ``// Function to calculate the minimum``    ``// elements removed``    ``public` `static` `int` `reduceArrSize(``int``[] arr)``    ``{``        ``Dictionary<``int``, ``int``> hm = ``new` `Dictionary<``int``, ``int``>();` `        ``// Making frequency map of elements``        ``for` `(``int` `i = 0; i < arr.Length; i++) {``            ``if``(hm.ContainsKey(arr[i])){``                ``hm[arr[i]] = hm[arr[i]]+1;``            ``}``            ``else``{``                ``hm.Add(arr[i], 1);``            ``}``        ``}` `        ``// Storing frequencies in a list``        ``List<``int``> freq``            ``= ``new` `List<``int``>(hm.Values);` `        ``// Sorting the list``        ``freq.Sort();` `        ``int` `size = arr.Length;``        ``int` `idx = freq.Count - 1;``        ``int` `count = 0;` `        ``// Counting number of elements to be deleted``        ``while` `(size > arr.Length / 2) {``            ``size -= freq[idx--];``            ``count++;``        ``}``        ``return` `count;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``int``[] arr = { 2, 2, 2, 2, 4, 4,``                      ``6, 7, 7, 3, 3, 3 };``        ``int` `count = reduceArrSize(arr);``        ``Console.WriteLine(count);``    ``}``}` `// This code is contributed by 29AjayKumar`

Javascript

 ``

Output:

`2`

Time Complexity: O(N*logN), sorting the frequency list
Auxiliary Space: O(N), hashmap to store the frequencies

Another efficient approach(Using a Priority Queue):

We can solve the problem by using a priority queue instead of sorting the frequency of elements. So we initialize a priority queue and store the frequency of every unique element which we get from an unordered map. Then we run a while loop and start deleting the elements from the priority queue which has the highest frequency to the lowest frequencies in decreasing order and hence we get the count of elements to be deleted. The time complexity of this approach is O(n*logn) but it will take very less time if elements are more frequent.

Below is the implementation of the above approach:

C++

 `// C++ Program to implement the above approach``#include ``using` `namespace` `std;` `// Function to calculate the minimum``// elements removed``int` `reduceArrSize(``int` `arr[],``int` `n)``    ``{``        ``unordered_map<``int``,``int``> hm;` `        ``// Making frequency map of elements``        ``for` `(``int` `i = 0; i < n; i++) {``            ``hm[arr[i]]++;``        ``}``        ` `        ``priority_queue<``int``> pq;` `        ``for``(``auto` `it = hm.begin(); it != hm.end(); it++)``        ``{``            ``pq.push(it->second);``        ``}` `        ``int` `size = n;``        ``int` `count = 0;` `        ``// Counting number of elements to be deleted``        ``while` `(size > n/ 2) {``            ``size -= pq.top();``            ``pq.pop();``            ``count++;``        ``}``        ``return` `count;``    ``}` `// Driver Code``int` `main()``    ``{``        ``int` `arr[] = { 2, 2, 2, 2, 4, 4, 6, 7, 7, 3, 3, 3 };``        ``int` `n = ``sizeof``(arr)/``sizeof``(arr[0]);``        ``int` `count = reduceArrSize(arr, n);``        ``cout<<(count);``        ``return` `0;``    ``}` `// This code is contributed by Pushpesh raj`

Output

`2`

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

My Personal Notes arrow_drop_up