C BINARY TREE

#include <stdio.h>
#include <stdlib.h>

/* A basic unbalanced binary search tree implementation in C, with the following functionalities implemented:
 - Insertion
 - Deletion
 - Search by key value
 - Listing of node keys in order of value (from left to right)
*/

// Node, the basic data structure in the tree
typedef struct node{

    // left child
	struct node* left;

	// right child
	struct node* right;

	// data of the node
	int data;
} node;

// The node constructor, which receives the key value input and returns a node pointer
node* newNode(int data){

    // creates a slug
	node* tmp = (node*)malloc(sizeof(node));

	// initializes the slug
	tmp->data = data;
	tmp->left = NULL;
	tmp->right = NULL;

	return tmp;
}

// Insertion procedure, which inserts the input key in a new node in the tree
node* insert(node* root, int data){
	// If the root of the subtree is null, insert key here
	if (root == NULL)
		root = newNode(data);
	// If it isn't null and the input key is greater than the root key, insert in the right leaf
	else if (data > root->data)
		root->right = insert(root->right, data);
	// If it isn't null and the input key is lower than the root key, insert in the left leaf
	else if (data < root->data)
		root->left = insert(root->left, data);
	// Returns the modified tree
	return root;
}

// Utilitary procedure to find the greatest key in the left subtree
node* getMax(node* root){
	// If there's no leaf to the right, then this is the maximum key value
	if (root->right == NULL)
		return root;
	else
		root->right = getMax(root->right);
}

// Deletion procedure, which searches for the input key in the tree and removes it if present
node* delete(node* root, int data){
	// If the root is null, nothing to be done
	if (root == NULL)
		return root;
	// If the input key is greater than the root's, search in the right subtree
	else if (data > root->data)
		root->right = delete(root->right, data);
	// If the input key is lower than the root's, search in the left subtree
	else if (data < root->data)
		root->left = delete(root->left, data);
	// If the input key matches the root's, check the following cases
	// termination condition
	else if (data == root->data){
		// Case 1: the root has no leaves, remove the node
		if ((root->left == NULL) && (root->right == NULL)){
			free(root);
			return NULL;
		}
		// Case 2: the root has one leaf, make the leaf the new root and remove the old root
		else if (root->left == NULL){
			node* tmp = root;
			root = root->right;
			free(tmp);
			return root;
		}
		else if (root->right == NULL){
			node* tmp = root;
			root = root->left;
			free(tmp);
			return root;
		}
		// Case 3: the root has 2 leaves, find the greatest key in the left subtree and switch with the root's
		else {

            // finds the biggest node in the left branch.
			node* tmp = getMax(root->left);

			// sets the data of this node equal to the data of the biggest node (lefts)
			root->data = tmp->data;
			root->left = delete(root->left, tmp->data);
		}
	}
	return root;
}

// Search procedure, which looks for the input key in the tree and returns 1 if it's present or 0 if it's not in the tree
int find(node* root, int data){
	// If the root is null, the key's not present
	if (root == NULL)
		return 0;
	// If the input key is greater than the root's, search in the right subtree
	else if (data > root->data)
		return find(root->right, data);
	// If the input key is lower than the root's, search in the left subtree
	else if (data < root->data)
		return find(root->left, data);
	// If the input and the root key match, return 1
	else if (data == root->data)
		return 1;
}

// Utilitary procedure to measure the height of the binary tree
int height(node* root){
	// If the root is null, this is the bottom of the tree (height 0)
	if (root == NULL)
		return 0;
	else{
		// Get the height from both left and right subtrees to check which is the greatest
		int right_h = height(root->right);
		int left_h = height(root->left);

		// The final height is the height of the greatest subtree(left or right) plus 1(which is the root's level)
		if (right_h > left_h)
			return (right_h + 1);
		else
			return (left_h + 1);
	}
}

// Utilitary procedure to free all nodes in a tree
void purge(node* root){
	if (root != NULL){
		if (root->left != NULL)
			purge(root->left);
		if (root->right != NULL)
			purge(root->right);
		free(root);
	}
}

// Traversal procedure to list the current keys in the tree in order of value (from the left to the right)
void inOrder(node* root){
	if(root != NULL){
		inOrder(root->left);
		printf("\t[ %d ]\t", root->data);
		inOrder(root->right);
	}
}

void main(){

    // this reference don't change.
    // only the tree changes.
	node* root = NULL;
	int opt = -1;
	int data = 0;

    // event-loop.
	while (opt != 0){
		printf("\n\n[1] Insert Node\n[2] Delete Node\n[3] Find a Node\n[4] Get current Height\n[5] Print Tree in Crescent Order\n[0] Quit\n");
		scanf("%d",&opt); // reads the choice of the user

        // processes the choice
		switch(opt){
			case 1:	printf("Enter the new node's value:\n");
				scanf("%d",&data);
				root = insert(root,data);
				break;

			case 2: printf("Enter the value to be removed:\n");
				if (root != NULL){
					scanf("%d",&data);
					root = delete(root,data);
				}
				else
					printf("Tree is already empty!\n");
				break;

			case 3: printf("Enter the searched value:\n");
				scanf("%d",&data);
				find(root,data) ? printf("The value is in the tree.\n") : printf("The value is not in the tree.\n");
				break;

			case 4: printf("Current height of the tree is: %d\n", height(root));
				break;

			case 5: inOrder(root);
				break;
		}
	}

    // deletes the tree from the heap.
	purge(root);
}