How to generate Combinations without repetition recursively in C#

// 1: [0, 1]
// 2: [0, 2]
// 3: [0, 3]
// 4: [1, 2]
// 5: [1, 3]
// 6: [2, 3]

using System;

public static class CombinationsNoRep
{
    private static int numberofCombos;
    private static int n;
    private static int k;
    private static int[] storageArr;

    public static void Main()
    {
        n = 4;
        k = 2;
        storageArr = new int[k];

        GenCombinationsNoRep();
    }

    private static void GenCombinationsNoRep(int index = 0, int element = 0)
    {
        if (index >= storageArr.Length)
        {
            PrintCombo();
            return;
        }

        for (int i = element; i < n; i++)
        {
            storageArr[index] = i;
            GenCombinationsNoRep(index + 1, i + 1);
        }
    }

    private static void PrintCombo()
    {
            Console.WriteLine(
                "{0,3}: [{1}]", 
                ++numberofCombos, 
                string.Join(", ", storageArr));
    }
}

Generic BinaryHeap implementation with C#

namespace BinaryHeap
{
    using System;
    using System.Collections.Generic;

    public class BinaryHeap<T> where T : IComparable<T>
    {
        private IList<T> heap;

        public BinaryHeap(T[] elements = null)
        {
            if (elements != null)
            {
                this.heap = new List<T>(elements);
                for (int i = elements.Length / 2; i >= 0; i--)
                {
                    this.HeapifyDown(i);
                }
            }
            else
            {
                this.heap = new List<T>();
            }
        }

        public int Count
        {
            get
            {
                return this.heap.Count;
            }
        }

        public T ExtractMax()
        {
            var max = this.heap[0];
            this.heap[0] = this.heap[this.Count - 1];
            this.heap.RemoveAt(this.Count - 1);

            if (this.Count > 0)
            {
                this.HeapifyDown(0);
            }

            return max;
        }

        public T PeekMax()
        {
            var max = this.heap[0];

            return max;
        }

        public void Insert(T node)
        {
            this.heap.Add(node);

            this.HeapifyUp(this.Count - 1);
        }

        private void HeapifyDown(int i)
        {
            var leftChild = (i * 2) + 1;
            var rightChild = (i * 2) + 2;
            var biggest = i;

            if (leftChild < this.Count && this.heap[leftChild].CompareTo(this.heap[biggest]) > 0)
            {
                biggest = leftChild;
            }

            if (rightChild < this.Count && this.heap[rightChild].CompareTo(this.heap[biggest]) > 0)
            {
                biggest = rightChild;
            }

            if (biggest != i)
            {
                T old = this.heap[i];
                this.heap[i] = this.heap[biggest];
                this.heap[biggest] = old;
                this.HeapifyDown(biggest);
            }
        }

        private void HeapifyUp(int i)
        {
            var parent = (i - 1) / 2;
            while (i > 0 && this.heap[i].CompareTo(this.heap[parent]) > 0)
            {
                var temp = this.heap[parent];
                this.heap[parent] = this.heap[i];
                this.heap[i] = temp;
                i = parent;
                parent = (i - 1) / 2;
            }
        }
    }
}

Simple implementation of generic BINARY TREE in C#

using System;
public class BinaryTree
{
	public BinaryTree(
		T value,
		BinaryTree leftNode = null,
		BinaryTree rightNode = null)
	{
		this.Value = value;
		this.LeftNode = leftNode;
		this.RightNode = rightNode;
	}

	public T Value { get; private set; }

	public BinaryTree LeftNode { get; private set; }

	public BinaryTree RightNode { get; private set; }

	public void EachPreOrder(Action action)
	{
		action(this.Value);

		if (this.LeftNode != null)
		{
			this.LeftNode.EachPreOrder(action);
		}

		if (this.RightNode != null)
		{
			this.RightNode.EachPreOrder(action);
		}
	}

	public void EachInOrder(Action action)
	{
		if (this.LeftNode != null)
		{
			this.LeftNode.EachPreOrder(action);
		}

		action(this.Value);

		if (this.RightNode != null)
		{
			this.RightNode.EachPreOrder(action);
		}
	}

	public void EachPostOrder(Action action)
	{
		if (this.LeftNode != null)
		{
			this.LeftNode.EachPreOrder(action);
		}

		if (this.RightNode != null)
		{
			this.RightNode.EachPreOrder(action);
		}
		
		action(this.Value);
	}
}

Simple implementation of generic TREE in C#

using System;
using System.Collections.Generic;

public class Tree<T>
{
	public Tree(
		T value,
		params Tree<T>[] children)
	{
		this.Value = value;

		this.Children = new List<Tree<T>>();
		foreach (var child in children)
		{
			this.Children.Add(child);
		}
	}
		
	public T Value { get; private set; }

	public ICollection<Tree<T>> Children { get; private set; }

	public void EachTree(Action<T> action)
	{
		action(this.Value);

		foreach (var child in this.Children)
		{
			child.EachTree(action);
		}
	}

	public void PrintTree(int indent = 0)
	{
		Console.WriteLine(new string(' ', indent * 2) + this.Value);
		indent++;
		foreach (var child in this.Children)
		{
			child.PrintTree(indent);
		}
	}
}

Simple dynamic implementation of generic STACK in C#

using System;

public class LinkedStack<T>
{
	private LinkedStackNode<T> firstNode;

	public int Count { get; private set; }

	public void Push(T element)
	{
		var newNode = new LinkedStackNode<T>(element);

		newNode.NextNode = this.firstNode;
		this.firstNode = newNode;

		this.Count++;
	}

	public T Pop()
	{
		if (this.Count <= 0)
		{
			throw new InvalidOperationException("Stack is empty.");
		}

		var nodeToPop = this.firstNode;
		this.firstNode = this.firstNode.NextNode;
		this.Count--;

		return nodeToPop.Value;
	}

	public T Peek()
	{
		var nodeToPeek = this.firstNode;

		return nodeToPeek.Value;
	}

	public T[] ToArray()
	{
		var arr = new T[this.Count];
		var currentNode = this.firstNode;
		var arrIndex = 0;
		while (currentNode != null)
		{
			arr[arrIndex] = currentNode.Value;
			arrIndex++;
			currentNode = currentNode.NextNode;
		}

		return arr;
	}

	private class LinkedStackNode<T>
	{
		public LinkedStackNode(
			T value,
			LinkedStackNode<T> nextNode = null)
		{
			this.Value = value;
		}

		public T Value { get; private set; }

		public LinkedStackNode<T> NextNode { get; set; }
	}
}

Simple static implementation of generic STACK in C#

using System;

public class ArrayStack<T> : IArrayStack<T>
{
	private const int InitialCapacity = 16;

	private T[] internalStorage;
	private int capacity;

	public ArrayStack(int capacity = InitialCapacity)
	{
		this.Capacity = capacity;
		this.internalStorage = new T[this.Capacity];
	}

	public int Count { get; private set; }

	private int Capacity
	{
		get
		{
			return this.capacity;
		}

		set
		{
			if (value <= 0)
			{
				throw new ArgumentOutOfRangeException(
					nameof(value),
					"Capacity should be positive integer number.");
			}

			this.capacity = value;
		}
	}

	public void Push(T element)
	{
		if (this.GrowNeeded())
		{
			this.Grow();
		}

		this.internalStorage[this.Count] = element;
		this.Count++;
	}

	public T Pop()
	{
		if (this.Count <= 0)
		{
			throw new InvalidOperationException("Stack is empty.");
		}
		
		this.Count--;
		var element = this.internalStorage[this.Count];
		this.internalStorage[this.Count] = default(T);

		return element;
	}

	public T Peek()
	{
		var element = this.internalStorage[this.Count - 1];

		return element;
	}

	public T[] ToArray()
	{
		var arr = new T[this.Count];
		this.FillStorage(arr);
		Array.Reverse(arr);

		return arr;
	}

	private bool GrowNeeded()
	{
		var result = this.Count >= this.Capacity;

		return result;
	}

	private void Grow()
	{
		this.Capacity *= 2;
		var newStorage = new T[this.Capacity];
		this.FillStorage(newStorage);
		this.internalStorage = newStorage;
	}

	private void FillStorage(T[] array)
	{
		Array.Copy(this.internalStorage, array, this.Count);
	}
}

Simple dynamic implementation of generic QUEUE in C#

using System;

public class LinkedQueue<T>
{
	private LinkedQueueNode<T> start;
	private LinkedQueueNode<T> end;

	public LinkedQueue()
	{
		this.Count = 0;
	}

	public int Count { get; private set; }

	public void Enqueue(T element)
	{
		var newElement = new LinkedQueueNode<T>(element);

		if (this.Count == 0)
		{
			this.start = this.end = newElement;
		}
		else
		{
			this.end.Next = newElement;
			this.end = newElement;
		}

		this.Count++;
	}

	public T Dequeue()
	{
		if (this.Count <= 0)
		{
			throw new InvalidOperationException("Queue is empty.");
		}

		var elementToReturn = this.start;
		this.start = this.start.Next;
		this.Count--;

		return elementToReturn.Value;
	}

	public T Peak()
	{
		if (this.Count <= 0)
		{
			throw new InvalidOperationException("Queue is empty.");
		}

		var currentElement = this.start.Value;

		return currentElement;
	}

	public T[] ToArray()
	{
		var arrToReturn = new T[this.Count];
		var currentNode = this.start;
		var arrIndex = 0;
		while (currentNode != null)
		{
			arrToReturn[arrIndex] = currentNode.Value;
			arrIndex++;
			currentNode = currentNode.Next;
		}

		return arrToReturn;
	}
	
	private class LinkedQueueNode<T>
	{
		public LinkedQueueNode(T value)
		{
			this.Value = value;
		}

		public T Value { get; private set; }

		public LinkedQueueNode<T> Next { get; set; }
	}
}

Simple static implementation of generic QUEUE in C#

public class CircularQueue<T>
{
	private const int InitialCapacity = 16;

	private T[] storage;

	private int capacity;
	private int startIndex;
	private int endIndex;

	public CircularQueue(int capacity = InitialCapacity)
	{
		this.Capacity = capacity;
		this.storage = new T[this.Capacity];
	}

	public int Count { get; private set; }

	private int Capacity
	{
		get
		{
			return this.capacity;
		}

		set
		{
			if (value <= 0)
			{
				throw new ArgumentOutOfRangeException(
					nameof(value),
					"Queue capacity should be a positive integer");
			}

			this.capacity = value;
		}
	}

	public void Enqueue(T element)
	{
		if (this.GrowNeeded())
		{
			this.Grow();
		}

		this.storage[this.endIndex] = element;
		this.endIndex = (this.endIndex + 1) % this.Capacity;
		this.Count++;
	}

	public T Dequeue()
	{
		if (this.Count <= 0)
		{
			throw new InvalidOperationException("Queue is empty.");
		}

		var element = this.storage[this.startIndex];
		this.storage[this.startIndex] = default(T);
		this.startIndex = (this.startIndex + 1) % this.Capacity;
		this.Count--;

		return element;
	}

	public T[] ToArray()
	{
		var resultArray = new T[this.Count];
		this.ResizeArrayStorage(resultArray);

		return resultArray;
	}

	private bool GrowNeeded()
	{
		var result = this.Count >= this.Capacity;

		return result;
	}

	private void Grow()
	{
		var newStorage = new T[this.Capacity * 2];
		this.ResizeArrayStorage(newStorage);
		this.startIndex = 0;
		this.endIndex = this.Capacity;
		this.Capacity *= 2;

		this.storage = newStorage;
	}

	private void ResizeArrayStorage(T[] array)
	{
		for (int i = 0; i < this.Count; i++)
		{
			var currentIndex = (this.startIndex + i) % this.Capacity;
			array[i] = this.storage[currentIndex];
		}
	}
}

How to calculate factorial with single line of code in C#

using System;
using System.Linq;
using System.Numerics;

public static class Factorial
{
    public static void Main(string[] args)
    {
        int num = 100;
        Console.WriteLine(Enumerable.Range(1, num).Select(i => new BigInteger(i)).Aggregate((a, b) => a * b));
    }
}
// 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

How to generate Variations without repetition recursively in C#

using System;
using System.Linq;

class VariationsWithoutRepetition
{
    const int k = 2;
    const int n = 4;

    static int[] arr = new int[k];
    static int[] free = Enumerable.Range(1, 4).ToArray();

    static void Main()
    {
        GenerateVariationsNoRepetitions(0);
    }

    static void GenerateVariationsNoRepetitions(int index)
    {
        if (index >= k)
        {
            PrintVariations();
        }
        else
        {
            for (int i = index; i < n; i++)
            {
                arr[index] = free[i];
                Swap(ref free[i], ref free[index]);
                GenerateVariationsNoRepetitions(index + 1);
                Swap(ref free[i], ref free[index]);
            }
        }
    }

    private static void Swap<T>(ref T v1, ref T v2)
    {
        T old = v1;
        v1 = v2;
        v2 = old;
    }

    static void PrintVariations()
    {
        Console.WriteLine("(" + string.Join(", ", arr) + ")");
    }
}


// result:
// (1, 2)
// (1, 3)
// (1, 4)
// (2, 1)
// (2, 3)
// (2, 4)
// (3, 2)
// (3, 1)
// (3, 4)
// (4, 2)
// (4, 3)
// (4, 1)