Find K most occurring elements in the given Array

// C# implementation to find
// k elements with max occurrence.

using System;
using System.Collections.Generic;

public class Comparer : IComparer<KeyValuePair<int, int> > {
    public int Compare(KeyValuePair<int, int> p2,
                       KeyValuePair<int, int> p1)
    {
        // If frequencies of two elements are same
        // then the larger number should come first
        if (p1.Value == p2.Value)
            return p1.Key.CompareTo(p2.Key);

        // Sort on the basis of decreasing order
        // of frequencies
        return p1.Value.CompareTo(p2.Value);
    }
}

public class KFrequentNumbers {
    static void print_N_mostFrequentNumber(int[] arr, int N,
                                           int K)
    {

        IDictionary<int, int> mp
            = new Dictionary<int, int>();

        // Put count of all the
        // distinct elements in Map
        // with element as the key &
        // count as the value.
        for (int i = 0; i < N; i++) {

            // Get the count for the
            // element if already present in the
            // Map or get the default value which is 0
            if (mp.ContainsKey(arr[i]))
                mp[arr[i]] += 1;
            else
                mp[arr[i]] = 1;
        }

        // Create a list from elements of HashMap
        List<KeyValuePair<int, int> > list
            = new List<KeyValuePair<int, int> >();
        foreach(KeyValuePair<int, int> entry in mp)
        {
            list.Add(entry);
        }

        // Sort the list
        Comparer compare = new Comparer();
        list.Sort(compare);

        for (int i = 0; i < K; i++)
            Console.Write(list[i].Key + " ");
    }

    // Driver's code
    public static void Main(string[] args)
    {
        int[] arr = { 3, 1, 4, 4, 5, 2, 6, 1 };
        int N = arr.Length;
        int K = 2;

        Console.Write(
            K + " elements with most occurrences are:\n");

        // Function call
        print_N_mostFrequentNumber(arr, N, K);
    }
}

// this code is contributed by phasing17

// C# implementation to find k
// elements with max occurrence.

using System;
using System.Collections.Generic;
using System.Linq;

public class KFrequentNumbers {
    static void print_N_mostFrequentNumber(int[] arr, int N,
                                           int K)
    {
        Dictionary<int, int> mp
            = new Dictionary<int, int>();

        // Put count of all the
        // distinct elements in Map
        // with element as the key &
        // count as the value.
        for (int i = 0; i < N; i++) {

            // Get the count for the
            // element if already
            // present in the Map or
            // get the default value
            // which is 0.
            if (!mp.ContainsKey(arr[i]))
                mp[arr[i]] = 0;
            mp[arr[i]]++;
        }

        // Create a Priority Queue
        // to sort based on the
        // count or on the key if the
        // count is same
        List<int> queue = mp.Keys.ToList();
        queue.Sort(delegate(int y, int x) {
            if (mp[x] == mp[y])
                return x.CompareTo(y);
            else
                return (mp[x]).CompareTo(mp[y]);
        });

        // Print the top k elements
        Console.WriteLine(
            K + " numbers with the most occurrences are:");
        for (int i = 0; i < K; i++) {
            Console.WriteLine(queue[i] + " ");
        }
    }

    // Driver's Code
    public static void Main(string[] args)
    {
        int[] arr = { 3, 1, 4, 4, 5, 2, 6, 1 };
        int N = arr.Length;
        int K = 2;

        // Function call
        print_N_mostFrequentNumber(arr, N, K);
    }
}

// This code is contributed by phasing17

// C# program to find k numbers with most
// occurrences in the given array
using System;
using System.Collections.Generic;
class GFG {
    // Function to print the k numbers with most occurrences
    static void print_N_mostFrequentNumber(int[] arr, int N,
                                           int K)
    {
        // HashMap to store count of the elements
        Dictionary<int, int> elementCount
            = new Dictionary<int, int>();
        for (int i = 0; i < N; i++) {
            if (elementCount.ContainsKey(arr[i])) {
                elementCount[arr[i]]++;
            }
            else {
                elementCount.Add(arr[i], 1);
            }
        }

        // Array to store the elements according
        // to their frequency
        List<int>[] frequency = new List<int>[ N + 1 ];
        for (int i = 0; i <= N; i++) {
            frequency[i] = new List<int>();
        }
        // Inserting elements in the frequency array
        foreach(
            KeyValuePair<int, int> element in elementCount)
        {
            frequency[element.Value].Add(element.Key);
        }

        int count = 0;
        Console.WriteLine(
            K + " numbers with most occurrences are:");

        for (int i = N; i >= 0; i--) {

            //   if frequency is same,then take number with
            //   a
            // larger value
            // so,if more than 1 number present with same
            // frequency,then sort frequency[i] in
            // descending order
            if (frequency[i].Count > 1) {
                frequency[i].Sort();
                frequency[i].Reverse();
            }

            foreach(int element in frequency[i])
            {
                count += 1;
                Console.Write(element + " ");
                // if K elements have been printed
                if (count == K)
                    return;
            }
        }

        return;
    }

    // Driver's code
    public static void Main(string[] args)
    {
        int[] arr = { 3, 1, 4, 4, 5, 2, 6, 1 };
        int N = 8;
        int K = 2;

        // Function call
        print_N_mostFrequentNumber(arr, N, K);
    }
}

// This code is contributed by shubhamm050402

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    // Function to find k numbers with most occurrences in the given array
    static List<int> PrintNMostFrequentNumber(List<int> nums, int k)
    {
        // Dictionary for counting the number of occurrences
        Dictionary<int, int> counts = new Dictionary<int, int>();

        // Storing the frequency of each element
        foreach (int num in nums)
        {
            if (counts.ContainsKey(num))
                counts[num]++;
            else
                counts[num] = 1;
        }

        // Creating a list to store the frequency
        List<Tuple<int, int>> freqs = new List<Tuple<int, int>>();
        foreach (var kvp in counts)
        {
            freqs.Add(new Tuple<int, int>(kvp.Value, kvp.Key));
        }

        // Using the user-defined function (CustomNthElement) to extract the values
        CustomNthElement(freqs, k);

        // Using user-defined function Transform to make the desired changes
        List<int> outList = freqs.GetRange(freqs.Count - k, k).Select(t => t.Item2).ToList();

        // Return the result
        return outList;
    }

    // Custom implementation of nth_element
    static void CustomNthElement(List<Tuple<int, int>> freqs, int k)
    {
        int left = 0;
        int right = freqs.Count - 1;

        while (left <= right)
        {
            int pivot = Partition(freqs, left, right);
            if (pivot == freqs.Count - k)
                return;
            else if (pivot < freqs.Count - k)
                left = pivot + 1;
            else
                right = pivot - 1;
        }
    }

    // Partition function for the custom nth_element
    static int Partition(List<Tuple<int, int>> freqs, int left, int right)
    {
        int pivot = freqs[right].Item1;
        int i = left;

        for (int j = left; j < right; j++)
        {
            if (freqs[j].Item1 <= pivot)
            {
                Swap(freqs, i, j);
                i++;
            }
        }

        Swap(freqs, i, right);
        return i;
    }

    // Helper function to swap elements in the list
    static void Swap(List<Tuple<int, int>> freqs, int i, int j)
    {
        Tuple<int, int> temp = freqs[i];
        freqs[i] = freqs[j];
        freqs[j] = temp;
    }

    static void Main(string[] args)
    {
        List<int> arr = new List<int> { 3, 1, 4, 4, 5, 2, 6, 1 };
        int K = 2;

        // Function call
        List<int> ans = PrintNMostFrequentNumber(arr, K);
        Console.WriteLine(K + " numbers with most occurrences are : ");
        for (int i = ans.Count - 1; i >= 0; i--)
        {
            Console.Write(ans[i] + " ");
        }
    }
}