How to generate Variations with repetition interatively in C#

// (red, red)
// (red, green)
// (red, blue)
// (green, red)
// (green, green)
// (green, blue)
// (blue, red)
// (blue, green)
// (blue, blue)

using System;
using System.Linq;

public static class Program
{
    private static readonly string[] Fruits = { "red", "green", "blue"};

    public static void Main()
    {
        var k = 2;
        var n = 3;
        var arr = new int[k];

        while (true)
        {
            Console.WriteLine($"({string.Join(", ", arr.Select(e => Fruits[e]))})");

            var index = k - 1;
            while (index >= 0 && arr[index] == n - 1)
            {
                index--;
            }

            if (index < 0)
            {
                break;
            }

            arr[index]++;

            for (int i = index + 1; i < k; i++)
            {
                arr[i] = 0;
            }
        }
    }
}

How to generate Combinations without repetition interatively in C#

// n = 5
// k = 3
// 1, 2, 3
// 1, 2, 4
// 1, 2, 5
// 1, 3, 4
// 1, 3, 5
// 1, 4, 5
// 2, 3, 4
// 2, 3, 5
// 2, 4, 5
// 3, 4, 5

using System;
using System.Collections.Generic;

public static class GenerateCombinationsIteratively
{

    static void Main()
    {
        Console.Write("n = ");
        var n = int.Parse(Console.ReadLine());

        Console.Write("k = ");
        var k = int.Parse(Console.ReadLine());

        foreach (var combo in Combinations(k, n))
        {
            Console.WriteLine(string.Join(", ", combo));
        }
    }

    private static IEnumerable<int[]> Combinations(int k, int n)
    {
        var result = new int[k];
        var stack = new Stack<int>();
        stack.Push(1);

        while (stack.Count > 0)
        {
            var index = stack.Count - 1;
            var value = stack.Pop();

            while (value <= n)
            {
                result[index++] = value++;
                stack.Push(value);
                if (index == k)
                {
                    yield return result;
                    break;
                }
            }
        }
    }
}

How to generate Permutations without repetition iteratively in C#

// 1, 2, 3
// 2, 1, 3
// 3, 1, 2
// 1, 3, 2
// 2, 3, 1
// 3, 2, 1
// Total permutations: 6

using System;
using System.Linq;

public static class GeneratePermutationsIteratively
{
    public static void Main()
    {
        var num = int.Parse(Console.ReadLine());

        var numberOfPerm = 1;
        var elements = Enumerable.Range(1, num).ToArray();
        var workArr = Enumerable.Range(0, elements.Length + 1).ToArray();

        PrintPerm(elements);
        var index = 1;
        while (index < elements.Length)
        {
            workArr[index]--;
            var j = 0;
            if (index % 2 == 1)
            {
                j = workArr[index];
            }

            SwapInts(ref elements[j], ref elements[index]);
            index = 1;
            while (workArr[index] == 0)
            {
                workArr[index] = index;
                index++;
            }

            numberOfPerm++;
            PrintPerm(elements);
        }

        Console.WriteLine($"Total permutations: {numberOfPerm}");
    }

    private static void PrintPerm(int[] elements)
    {
        Console.WriteLine(string.Join(", ", elements));
    }

    private static void SwapInts(ref int a, ref int b)
    {
        a ^= b;
        b ^= a;
        a ^= b;
    }
}

How to generate Permutations with repetition recursively in C#

// 1: [6, 1, 1, 1, 1, 1, 1, 1, 1]
// 2: [1, 6, 1, 1, 1, 1, 1, 1, 1]
// 3: [1, 1, 6, 1, 1, 1, 1, 1, 1]
// 4: [1, 1, 1, 6, 1, 1, 1, 1, 1]
// 5: [1, 1, 1, 1, 6, 1, 1, 1, 1]
// 6: [1, 1, 1, 1, 1, 6, 1, 1, 1]
// 7: [1, 1, 1, 1, 1, 1, 6, 1, 1]
// 8: [1, 1, 1, 1, 1, 1, 1, 6, 1]
// 9: [1, 1, 1, 1, 1, 1, 1, 1, 6]

using System;

public static class PermutationsWithRep
{
    private static int numberOfCombos;

    public static void Main()
    {
        var collection = new[] { 6, 1, 1, 1, 1, 1, 1, 1, 1 };
        PermuteRep(collection);
    }

    private static void PermuteRep<T>(T[] workArray, int? end = null, int start = 0)
        where T : IComparable<T>
    {
        if (end == null)
        {
            end = workArray.Length - 1;
        }

        PrintPerm(workArray);

        for (int left = end.Value - 1; left >= start; left--)
        {
            for (int right = left + 1; right <= end; right++)
                if (workArray[left].CompareTo(workArray[right]) != 0)
                {
                    Swap(ref workArray[left], ref workArray[right]);
                    PermuteRep(workArray, end, left + 1);
                }

            var firstElement = workArray[left];

            for (int i = left; i <= end.Value - 1; i++)
            {
                workArray[i] = workArray[i + 1];
            }

            workArray[end.Value] = firstElement;
        }
    }

    private static void Swap<T>(ref T a, ref T b)
    {
        var temp = a;
        a = b;
        b = temp;
    }

    private static void PrintPerm<T>(T[] arr)
    {
        Console.WriteLine($"{++numberOfCombos}: [{string.Join(", ", arr)}]");
    }
}

How to generate Variations with repetition recursively in C#

// 1: [0, 0]
// 2: [0, 1]
// 3: [0, 2]
// 4: [1, 0]
// 5: [1, 1]
// 6: [1, 2]
// 7: [2, 0]
// 8: [2, 1]
// 9: [2, 2]

using System;

public static class VariationsWithRep
{
    private static int numberOfCombos;
    private static int n;
    private static int k;
    private static int[] workArr;

    public static void Main()
    {
        n = 3;
        k = 2;
        workArr = new int[k];

        GenerateVariationsWithRep();
    }

    private static void GenerateVariationsWithRep(int index = 0)
    {
        if (index >= k)
        {
            PrintCombo();
            return;
        }

        for (int i = 0; i < n; i++)
        {
            workArr[index] = i;
            GenerateVariationsWithRep(index + 1);
        }
    }

    private static void PrintCombo()
    {
        Console.WriteLine($"{++numberOfCombos}: [{string.Join(", ", workArr)}]");
    }
}

How to generate Combinations with repetition recursively in C#

//  1: [0, 0]
//  2: [0, 1]
//  3: [0, 2]
//  4: [0, 3]
//  5: [1, 1]
//  6: [1, 2]
//  7: [1, 3]
//  8: [2, 2]
//  9: [2, 3]
// 10: [3, 3]

using System;

public static class CombinationsWithRep
{
    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];

        GenCombinationsWithRep();
    }

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

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

    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);
		}
	}
}

How to convert letter char to its alphabet integer position in Java

public class LetterToAlphabetPos {
    public static void main(String[] args) {
        char letter = 'Z';
        System.out.println(letterToAlphabetPos(letter)); // returns 26
    }

    private static int letterToAlphabetPos(char letter) {
        return Character.toUpperCase(letter) - 64;
    }
}