Maximum distinct elements after removing k elements

Given an array arr[] containing n elements. The problem is to find the maximum number of distinct elements (non-repeating) after removing k elements from the array. 
Note: 1 <= k <= n.
Examples: 

Input : arr[] = {5, 7, 5, 5, 1, 2, 2}, k = 3
Output : 4
Remove 2 occurrences of element 5 and
1 occurrence of element 2.

Input : arr[] = {1, 2, 3, 4, 5, 6, 7}, k = 5
Output : 2

Input : arr[] = {1, 2, 2, 2}, k = 1
Output : 1

Approach: Following are the steps: 

1. Make a multi set from the given array.

2. During making this multiset check if the current element is present or not in multiset, if it is already present then simply reduce the k value and do not insert in the multiset.

3. If k becomes 0 then simply just put values in multiset.

4. After traversing the whole given array, 

       a) if k is not equal to zero then it means the multiset is consist of only unique elements and we have to remove any of the k elements from the multiset to make k=0, so in this case the answer will be size of multiset minus k value at that time.

        b) if k is equal to zero then it means there may be duplicate values present in the multiset so put all the values in a set and the size of this set will be the number of distinct elements after removing k elements

C++

// CPP implementation of the above approach
#include <bits/stdc++.h>
using namespace std;

// function to find maximum distinct elements
// after removing k elements
int maxDistinctNum(int a[], int n, int k)
{
  int i;
  multiset<int> s;
  // making multiset from given array
        for(i=0;i<n;i++){
            if(s.find(a[i])==s.end()||k==0)
            s.insert(a[i]);
            else
            {
                k--;
            }
        }

        if(k!=0)
        return s.size()-k;
        else{
            set<int> st;
            for(auto it:s){
                st.insert(it);
            }
            return st.size();
        }
}

// Driver Code
int main()
{
    int arr[] = { 5, 7, 5, 5, 1, 2, 2 };
    int n = sizeof(arr) / sizeof(arr[0]);
    int k = 3;

    // Function Call
    cout << "Maximum distinct elements = "
         << maxDistinctNum(arr, n, k);
    return 0;
}

Python3

# Python implementation of the above approach

# function to find maximum distinct elements after removing k elements
def maxDistinctNum(a, n, k):

   # making multiset from given array multisets are like dictionaries ,
   # so will initialise a dictionary
    s = {}
    for i in range(n):
        if a[i] not in s or k == 0:
            s[a[i]] = s.get(a[i], 0)+1
        else:
            s[a[i]] = 1
            k -= 1
    if k != 0:
        return len(s)-k
    else:

        st = set()
        for i in s:
            st.add(i)
        return len(st)

# Driver Code
if __name__ == "__main__":

  # Array
    arr = [5, 7, 5, 5, 1, 2, 2]
    K = 3

    # Size of array
    N = len(arr)

    # Function Call
    print("Maximum distinct elements = ", maxDistinctNum(arr, N, K))

# This code is contributed by vivekmaddheshiya205

Java

// Java implementation of the
// above approach
import java.util.*;
class GFG{

// Function to find maximum
// distinct elements after
// removing k elements
static int maxDistinctNum(int arr[],
                          int n, int k)
{
  HashMap<Integer,
          Integer> numToFreq = new HashMap<>();

  // Build frequency map
  for(int i = 0 ; i < n ; i++)
  {
    numToFreq.put(arr[i],
    numToFreq.getOrDefault(arr[i], 0) + 1);
  }

  int result = 0;

  // Min-heap
  PriorityQueue<Integer> minHeap =
                new PriorityQueue<Integer>();

  // Add all number with freq=1 to
  // result and push others to minHeap
  for(Map.Entry<Integer,
                Integer> p : numToFreq.entrySet())
  {
    if(p.getValue() == 1)
      ++result;
    else
      minHeap.add(p.getValue());
  }

  // Perform k operations
  while(k != 0 && !minHeap.isEmpty())
  {
    // Pop the top() element
    Integer t = minHeap.poll();

    // Increment Result
    if(t == 1)
    {
      ++result;
    }

    // Reduce t and k
    // Push it again
    else
    {
      --t;
      --k;
      minHeap.add(t);
    }
  }

  // Return result
  return result;
}

// Driver code
public static void main(String[] args)
{
  int arr[] = {5, 7, 5, 5, 1, 2, 2};
  int n = arr.length;
  int k = 3;

  // Function Call
  System.out.println("Maximum distinct elements = " +
                      maxDistinctNum(arr, n, k));
}
}

// This code is contributed by rutvik_56

Javascript

<script>

// Javascript implementation of the above approach

// function to find maximum distinct elements
// after removing k elements
function maxDistinctNum(a, n, k)
{
  var i;
  var s = [];
  // making multiset from given array
        for(i=0;i<n;i++){
            if(!s.includes(a[i])||k==0)
            s.push(a[i]);
            else
            {
                k--;
            }
        }

        if(k!=0)
            return s.size-k;
        else{
            var st = new Set();
            s.forEach(element => {
                st.add(element);
            });

            return st.size;
        }
}

// Driver Code
var arr = [5, 7, 5, 5, 1, 2, 2];
var n = arr.length;
var k = 3;

// Function Call
document.write( "Maximum distinct elements = "
      +  maxDistinctNum(arr, n, k));

// This code is contributed by itsok.
</script>

Output

Maximum distinct elements = 4

Time Complexity: O(k*logd), where d is the number of distinct elements in the given array.

Another Approach: Follow the below steps, to solve this problem:

• Find the Number of distinct Toys.
• Sum of number of element except one element form every distinct Toys.
• Check sum if greater than or equal K then Return all distinct element.
• Otherwise decrement number of distinct element and to fill K.
• Return Size of vector.

Below is the implementation of the above approach:

C++

// C++ code for the above approach
#include <bits/stdc++.h>
using namespace std;
// function to return maximum number of distinct Toys
int MaxNumber(int arr[], int N, int K)
{
    // Count Number of distinct Number
    unordered_map<int, int> mp;
    for (int i = 0; i < N; i++) {
        mp[arr[i]]++;
    }
    // push them into vector
    vector<int> v1;
    for (auto i : mp) {
        v1.push_back(i.second);
    }
    // add number of element except one element from every
    // distinct element
    int temp = 0;
    for (int i = 0; i < v1.size(); i++) {
        temp += v1[i] - 1;
    }
    // check if it is greater than simply return size of
    // vector otherwise decrement size of vector to fill k
    if (K <= temp) {
        return v1.size();
    }
    else {
        K = K - temp;
        int ans = v1.size();
        while (K) {
            ans--;
            K--;
        }
        return ans;
    }
}
// Driver Code
int main()
{
    // array
    int arr[] = { 10, 10, 10, 50, 50 };
    int K = 3;
    // size of array
    int N = sizeof(arr) / sizeof(arr[0]);
    cout << MaxNumber(arr, N, K) << endl;
    return 0;
}

Java

// Java code for the above approach
import java.io.*;
import java.util.*;

class GFG {

  // Function to return maximum number of distinct Toys
  static int MaxNumber(int[] arr, int N, int K)
  {

    // Count Number of distinct Number
    HashMap<Integer, Integer> mp = new HashMap<>();
    for (int i = 0; i < N; i++) {
      mp.put(arr[i], mp.getOrDefault(arr[i], 0) + 1);
    }

    // pust them into arraylist
    List<Integer> v1 = new ArrayList<>();
    for (Map.Entry<Integer, Integer> i :
         mp.entrySet()) {
      v1.add(i.getValue());
    }

    // add number of element except one element from
    // every distinct element
    int temp = 0;
    for (int i = 0; i < v1.size(); i++) {
      temp += v1.get(i) - 1;
    }

    // check if it is greater than simply return size of
    // vector otherwise decrement size of vector to fill
    // k
    if (K <= temp) {
      return v1.size();
    }
    else {
      K = K - temp;
      int ans = v1.size();
      while (K != 0) {
        ans--;
        K--;
      }
      return ans;
    }
  }

  public static void main(String[] args)
  {
    int arr[] = { 10, 10, 10, 50, 50 };
    int K = 3;
    int N = arr.length;

    System.out.println(MaxNumber(arr, N, K));
  }
}

// This code is contributed by lokeshmvs21.

Python3

# Python3 code for the above approach

# function to return maximum number of distinct Toys
def MaxNumber(arr, N, K):

    # Count Number of distinct Number
    mp = {}
    for i in range(N):
        if arr[i] not in mp:
            mp[arr[i]] = 0
        mp[arr[i]] += 1

        # push them into vector
    v1 = []
    for i in mp:
        v1.append(mp[i])

     # add number of element except one element from every
    # distinct element
    temp = 0
    for i in range(len(v1)):
        temp += v1[i]-1

     # check if it is greater than simply return size of
    # vector otherwise decrement size of vector to fill k
    if K <= temp:
        return len(v1)
    else:
        K = K-temp
        ans = len(v1)
        while K:
            ans -= 1
            K -= 1
        return ans

# Driver Code
if __name__ == "__main__":

  # Array
    arr = [10, 10, 10, 50, 50]
    K = 3

    # Size of array
    N = len(arr)
    print(MaxNumber(arr, N, K))

    # This code is contributed by vivekmaddheshiya205

Output

2

Time Complexity: O(N)

Space Complexity: O(N)