now bfs using queues

lectures/20_trees_III/README.md

@@ -62,28 +62,39 @@ We can also implement operator++ for the ds_set iterator without using the paren
 - Write an algorithm to print the nodes in the tree one tier at a time, that is, in a breadth-first manner.
 
 ```cpp
-  void breadth_first_traverse(Node* root)
-  {
-    int level=0;
-    std::vector<Node*> current_level;
-    std::vector<Node*> next_level;
-    if(root==NULL){return;}
-    current_level.push_back(root);
-    while(current_level.size()!=0)
-    {
-      std::cout<<"level"<<level<<":";
-      for (unsigned i=0; i<current_level.size();i++)
-       {
-         if(current_level[i]->left != NULL)
-          next_level.push_back(current_level[i]->left);
-         if(current_level[i]->right != NULL)
-          next_level.push_back(current_level[i]->right);
-         std::cout<<" "<<current_level[i]->value;
+// the breadth-first traversal function using std::queue
+void breadth_first_traverse(Node* root) {
+    if (root == NULL) {
+        return;
     }
-       current_level = next_level;
+
+    std::queue<Node*> node_queue; // queue to store nodes for BFS traversal
+    node_queue.push(root);  // start by pushing the root node
+
+    int level = 0;
+
+    while (!node_queue.empty()) {
+        int level_size = node_queue.size();  // number of nodes at the current level
+        std::cout << "level " << level << ": ";
+
+        for (int i = 0; i < level_size; i++) {
+            Node* current_node = node_queue.front();  // get the front node
+            node_queue.pop();  // remove the node from the queue
+
+            std::cout << current_node->value << " ";  // print the value of the node
+
+            // push the children of the current node to the queue (if they exist)
+            if (current_node->left != NULL) {
+                node_queue.push(current_node->left);
+            }
+            if (current_node->right != NULL) {
+                node_queue.push(current_node->right);
+            }
+        }
+        // after we finish the for loop, the only pointers in the queue, are the pointers pointing to nodes of the next level.
+
+        std::cout << std::endl;
         level++;
-       next_level.clear();
-       std::cout<<std.endl;
     }
 }
 ```

lectures/20_trees_III/bfs_main.cpp

@@ -1,5 +1,5 @@
 #include <iostream>
-#include <vector>
+#include <queue>
 
 class Node {
 public:
@@ -11,33 +11,39 @@ public:
     Node(int val) : value(val), left(NULL), right(NULL) {}
 };
 
-// The breadth-first traversal function you provided
+// the breadth-first traversal function using std::queue
 void breadth_first_traverse(Node* root) {
-    int level = 0;
-    std::vector<Node*> current_level;
-    std::vector<Node*> next_level;
-
     if (root == NULL) {
         return;
     }
 
-    current_level.push_back(root);
+    std::queue<Node*> node_queue; // queue to store nodes for BFS traversal
+    node_queue.push(root);  // start by pushing the root node
 
-    while (current_level.size() != 0) {
+    int level = 0;
+    
+    while (!node_queue.empty()) {
+        int level_size = node_queue.size();  // number of nodes at the current level
         std::cout << "level " << level << ": ";
-        for (unsigned i = 0; i < current_level.size(); i++) {
-            if (current_level[i]->left != NULL) {
-                next_level.push_back(current_level[i]->left);
+
+        for (int i = 0; i < level_size; i++) {
+            Node* current_node = node_queue.front();  // get the front node
+            node_queue.pop();  // remove the node from the queue
+
+            std::cout << current_node->value << " ";  // print the value of the node
+
+            // push the children of the current node to the queue (if they exist)
+            if (current_node->left != NULL) {
+                node_queue.push(current_node->left);
             }
-            if (current_level[i]->right != NULL) {
-                next_level.push_back(current_level[i]->right);
+            if (current_node->right != NULL) {
+                node_queue.push(current_node->right);
             }
-            std::cout << " " << current_level[i]->value;
         }
-        current_level = next_level;
-        level++;
-        next_level.clear();
+	// after we finish the for loop, the only pointers in the queue, are the pointers pointing to nodes of the next level.
+
         std::cout << std::endl;
+        level++;
     }
 }
 

lectures/20_trees_III/bfs_main_vector.cpp

@@ -0,0 +1,72 @@
+#include <iostream>
+#include <vector>
+
+class Node {
+public:
+    int value;
+    Node* left;
+    Node* right;
+
+    // constructor to create a new node
+    Node(int val) : value(val), left(NULL), right(NULL) {}
+};
+
+// bfs using vectors
+void breadth_first_traverse(Node* root) {
+    int level = 0;
+    std::vector<Node*> current_level;
+    std::vector<Node*> next_level;
+
+    if (root == NULL) {
+        return;
+    }
+
+    current_level.push_back(root);
+
+    while (current_level.size() != 0) {
+        std::cout << "level " << level << ":";
+        for (unsigned i = 0; i < current_level.size(); i++) {
+            if (current_level[i]->left != NULL) {
+                next_level.push_back(current_level[i]->left);
+            }
+            if (current_level[i]->right != NULL) {
+                next_level.push_back(current_level[i]->right);
+            }
+            std::cout << " " << current_level[i]->value;
+        }
+        current_level = next_level;
+        level++;
+        next_level.clear();
+        std::cout << std::endl;
+    }
+}
+
+int main() {
+    // creating a simple binary tree
+    //     1
+    //   /   \
+    //  2     3
+    // / \   / \
+    //4   5 6   7
+    Node* root = new Node(1);
+    root->left = new Node(2);
+    root->right = new Node(3);
+    root->left->left = new Node(4);
+    root->left->right = new Node(5);
+    root->right->left = new Node(6);
+    root->right->right = new Node(7);
+
+    // calling the breadth-first traversal function
+    breadth_first_traverse(root);
+
+    // cleaning up dynamically allocated memory
+    delete root->left->left;
+    delete root->left->right;
+    delete root->right->left;
+    delete root->right->right;
+    delete root->left;
+    delete root->right;
+    delete root;
+
+    return 0;
+}