DataStructuresCSharp/C#/Datastructures/BinaryTree/Tree.cs

using C_.Datastructures.LinkedList;
using C_.Datastructures.Stack;
using C_.Datastructures.Queue;
using System;

namespace C_.Datastructures.BinaryTree
{
    internal class Tree<T> where T:IComparable
    {
        public TreeNode<T>? Root { get; set; }
        public int Count { get; set; }

        public static Tree<T> Create(){
            //Create a new Tree with no Head
            return new Tree<T>{
                Root = null,
                Count = 0
            };
        }

        public static Tree<T> Create(T value){
            //Create a new Tree with Head
            return new Tree<T>{
                Root = TreeNode<T>.Create(value)
            };
        }

        public void Add(T value)
        {//Add item to the correct position in the tree (Input cannot be null)
            Count++;

            if (Root == default)
            {//If new node should become the root
                Root = TreeNode<T>.Create(value);
                return;
            }

            //Find position to insert
            TreeNode<T> node = Root;
            node = Descend(value, node)!;

            if (value.CompareTo(node.Value) < 0)
            {//Insert to left
                node.Left = TreeNode<T>.Create(value);
                return;
            }
            //Insert to right
            node.Right = TreeNode<T>.Create(value);
            return;
        }

        public bool Delete(T value)
        {
            //Check if root of tree is null
            if (Count == 0)
                return false;
            
            //Check if the only value is the root
            if (Count == 1)
            {
                if (Root!.Value!.Equals(value))
                {//If the only item is the one we are trying to delete
                    Count = 0;
                    Root = default;
                    return true;
                }
                return false;
            }

            //Stack to store the items leading up to and including the one we are trying to delete    
            Stack<TreeNode<T>>? deletionStack;

            //Search for item being deleted + Parents
            deletionStack = Find(value);

            if (deletionStack == default)
                return false; //Item was not found

            //Delete Item (replace) and replace pointers to retain integrity of tree

            TreeNode<T>? node = deletionStack.Pop();

            //stack to store the items leading up to the value that we will use to replace the node
            Stack<TreeNode<T>>? replacementStack = Min(node!.Right);
            
            if (replacementStack == default)
            {//Nothing to the right of the value we are deleting
                if (deletionStack.Peek() != default)
                {//Parent adopts left hand side of node if present
                    deletionStack.Pop()!.Left = node.Left;
                }

                if (node.Left != default)
                {//Node adopts left value if no lower value to the right
                    node.Value = node.Left!.Value;
                    node.Left = node.Left.Left;
                }
            }
            else
            {//Replace the value + reorder nodes
                node.Value = replacementStack.Peek()!.Value;
                TreeNode<T>? replacementNode = replacementStack.Pop();

                switch (replacementStack.GetCount())
                {//Determine what to do based on number of items in replacement stack
                    case 1:
                        node.Right = replacementNode!.Right;
                        break;
                    case >=2:
                        replacementStack.Peek()!.Left = replacementNode!.Right;
                        break;
                    default:
                        break;
                }
            }
            Count--;
            return true;
        }

        public LinkedList<T>? Traverse(TraversalType traversalType)
        {
            if (Root == default)
                return default;

            LinkedList<T> list = LinkedList<T>.Create();

            switch (traversalType)
            {//Select which type of traversal to do
                case TraversalType.Inorder:
                    Inorder(list, Root);
                    break;
                case TraversalType.Preorder:
                    Preorder(list, Root);
                    break;
                case TraversalType.Postorder:
                    Postorder(list, Root);
                    break;
                case TraversalType.Breadth:
                    Queue<TreeNode<T>> queue = Queue<TreeNode<T>>.Create();
                    BreadthFirst(list, queue, Root);
                    break;
                default:
                    return default;
            }

            return list;
        }
        private void Inorder(LinkedList<T> list, TreeNode<T> node)
        {//Inorder Traversal
            if (node.Left != default)
                Inorder(list, node.Left);

            list.Append(node.Value);

            if (node.Right != default)
                Inorder(list, node.Right);
        }

        private void Preorder(LinkedList<T> list, TreeNode<T> node)
        {//Preorder Traversal
            list.Append(node.Value);
            
            if (node.Left != default)
                Preorder(list, node.Left);

            if (node.Right != default)
                Preorder(list, node.Right);
        }

        private void Postorder(LinkedList<T> list, TreeNode<T> node)
        {//Postorder Traversal
            if (node.Left != default)
                Postorder(list, node.Left);

            if (node.Right != default)
                Postorder(list, node.Right);

            list.Append(node.Value);
        }

        private void BreadthFirst(LinkedList<T> list, Queue<TreeNode<T>> queue, TreeNode<T> node)
        {//Breadth First Traversal
            list.Append(node.Value);

            if (node.Left != default)
                queue.Push(node.Left);

            if (node.Right != default)
                queue.Push(node.Right);

            //Only continue to traverse if there are no mode nodes to process
            if (queue.Peek() != default)
                BreadthFirst(list, queue, queue.Pop()!);
        }




        private static TreeNode<T>? GetNext(T value, TreeNode<T>? node)
        {//T is comparable so use methods to determine which way to traverse
            if(node == default)
                return default;

            if (value.CompareTo(node.Value) < 0)
            {//Traverse Left
                return node.Left;
            }
            //Traverse Right
            return node.Right;
        }

        private Stack<TreeNode<T>>? Find(T value)
        {//Return true if the item can be found within the tree
            if (Root == default || Root.Value!.Equals(default))
                return default;

            Stack<TreeNode<T>>? stack = Stack<TreeNode<T>>.Create(Root);

            while (stack.Peek() != default)
            {
                //Compare value at node to see if we are looking for the root item
                if (stack.Peek()!.Value!.Equals(value))
                    return stack;

                stack.Push(GetNext(value, stack.Peek()));
            }
            return default;
        }

        private static Stack<TreeNode<T>>? Min(TreeNode<T>? node)
        {//Returns a Stack with the value on top being the minimum of the subtree
            
            if(node == default)
                return default;

            //Stack to store and be able to get the parent values
            Stack<TreeNode<T>> stack = Stack<TreeNode<T>>.Create(node);

            while(true){
                if (stack.Peek()!.Left == default)
                    return stack;

                stack.Push(stack.Peek()!.Left);
            }
        }

        private static Stack<TreeNode<T>>? Max(TreeNode<T>? node)
        {///Returns a Stack with the value on top being the maximum of the subtree
            if(node == default)
                return default;

            //Stack to store and be able to get the parent values
            Stack<TreeNode<T>> stack = Stack<TreeNode<T>>.Create(node);

            while(true){
                if (stack.Peek()!.Right == default)
                    return stack;

                stack.Push(stack.Peek()!.Right);
            }
        }

        private TreeNode<T>? Descend(T value, TreeNode<T>? current)
        {//Keep trying to determine whether to go left or right until null node is found that can be appended to
            if (current == default)
                return default;

            TreeNode<T>? node;

            node = Descend(value, GetNext(value, current));
            if (node == null)
            {
                return current;
            }
            return node;
        }
    }

    public enum TraversalType
    {//Enum to allow for Traversal selection
        Inorder,
        Preorder,
        Postorder,
        Breadth
    }
}