Update binary_search_tree.c

data_structures/binary_trees/binary_search_tree.c

@@ -1,12 +1,21 @@
 #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{
 	struct node* left;
 	struct node* right;
 	int data;
 } node;
 
+// The node constructor, which receives the key value input and returns a node pointer
 node* newNode(int data){
 	node* tmp = (node*)malloc(sizeof(node));
 	tmp->data = data;
@@ -16,51 +25,62 @@ node* newNode(int data){
 	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
 	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->right;
-			free(root);
-			return tmp;
+			node* tmp = root;
+			root = root->right;
+			free(tmp);
+			return root;
 		}
 		else if (root->right == NULL){
-			node* tmp = root->left;
-			free(root);
-			return tmp;
+			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 {
 			node* tmp = getMax(root->left);
 			root->data = tmp->data;
@@ -70,24 +90,33 @@ node* delete(node* root, int 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
@@ -95,6 +124,7 @@ int height(node* root){
 	}
 }
 
+// Utilitary procedure to free all nodes in a tree
 void purge(node* root){
 	if (root != NULL){
 		if (root->left != NULL)
@@ -105,6 +135,7 @@ void purge(node* 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);