Merge pull request #389 from LuckyPigeon/bst_explanation

Add explanation of bst.cpp

Author: matthewsamuel95

BST/bst.cpp

@@ -3,56 +3,82 @@
 #include <list>
 using namespace std;
 
-list < int > arr;
+// Create binary searching tree structure
 struct node {
   int val;
   struct node * left, * right;
 };
+
+// Insert a number n to node's val, and pointer node's left and right
+// branch to NULL
 struct node * insert(int n) {
   struct node * temp = new struct node();
   temp -> val = n;
   temp -> left = NULL;
   temp -> right = NULL;
   return temp;
 }
+
+// Inorder permutation, permutate from left to root to right,
+// accordingly
 void inorder(struct node * root) {
   if (root != NULL) {
     inorder(root -> left);
     cout << root -> val << " ";
-    arr.push_back(root -> val);
     inorder(root -> right);
   }
 }
+
+// Postorder permutation, permutate from left to right to root,
+// accoringly
 void postorder(struct node * root) {
   if (root != NULL) {
     postorder(root -> right);
     postorder(root -> left);
     cout << root -> val << " ";
   }
 }
+
+// Preorder permutation, permutate from root to left to right,
+// accorginly
 void preorder(struct node * root) {
   if (root != NULL) {
     cout << root -> val << " ";
     preorder(root -> left);
     preorder(root -> right);
   }
 }
+
+// Create a new binary searching tree structure
 struct node * create(struct node * root, int n) {
+  // If root(node) is NULL, then insert a value n as it's value
   if (root == NULL)
     insert(n);
+  // Find the node that is NULL, else if val > n, then check root's left
+  // node is empty or not, else if val < n, then check root's right node
+  // is empty or not
   else if (root -> val > n) {
     root -> left = create(root -> left, n);
   } else
     root -> right = create(root -> right, n);
 }
 
+// Find a specific node that the val is equal to key
 bool find(struct node * root, int key) {
+  // If val > key and the next left node is NULL, return not found(0)
   if (root -> val > key && root -> left == NULL)
     return 0;
+	// If val < key and the next right node is NULL, return not found(0)
   if (root -> val < key && root -> right == NULL)
     return 0;
+	// If val = key then return found(1)
   if (root -> val == key)
     return 1;
+	// If val > key and the next left node is not NULL, then go to next
+	// left node and keep searching
+	// If val < key and the next right node is not NULL, then go to next
+	// right node and keep searching
+	// Else return not found(0)
   else if (root -> val > key) {
     find(root -> left, key);
   } else if (root -> val < key) {
@@ -61,7 +87,9 @@ bool find(struct node * root, int key) {
     return 0;
 }
 int main() {
+	// Init a empty node bst
   struct node * bst = NULL;
+	// Input each value of bst
   bst = create(bst, 5);
   create(bst, 10);
   create(bst, 20);
@@ -71,10 +99,13 @@ int main() {
   create(bst, 3);
   create(bst, 12);
   create(bst, 21);
+	// Inorder permutate from left -> root -> right
   inorder(bst);
   cout << endl;
+	// Postorder permutate from left -> right -> root
   postorder(bst);
   cout << endl;
+	// Preorder permutate from root -> left -> right
   preorder(bst);
   cout << endl;