diff --git a/lectures/19_trees_II/ds_set.h b/lectures/19_trees_II/ds_set.h
index fca846f..2697601 100644
--- a/lectures/19_trees_II/ds_set.h
+++ b/lectures/19_trees_II/ds_set.h
@@ -23,14 +23,14 @@ class tree_iterator {
 public:
   tree_iterator() : ptr_(NULL) {}
   tree_iterator(TreeNode<T>* p) : ptr_(p) {}
-  tree_iterator(const tree_iterator& old) : ptr_(old.ptr_) {}
+  tree_iterator(const tree_iterator& other) : ptr_(other.ptr_) {}
   ~tree_iterator() {}
-  tree_iterator& operator=(const tree_iterator& old) { ptr_ = old.ptr_;  return *this; }
+  tree_iterator& operator=(const tree_iterator& other) { ptr_ = other.ptr_;  return *this; }
   // operator* gives constant access to the value at the pointer
   const T& operator*() const { return ptr_->key; }
   // comparison operators are straightforward
-  bool operator== (const tree_iterator& rgt) { return ptr_ == rgt.ptr_; }
-  bool operator!= (const tree_iterator& rgt) { return ptr_ != rgt.ptr_; }
+  bool operator== (const tree_iterator& other) { return ptr_ == other.ptr_; }
+  bool operator!= (const tree_iterator& other) { return ptr_ != other.ptr_; }
   // increment & decrement operators
   tree_iterator<T> & operator++() { 
     // if i have right subtree, find left most element of those
@@ -47,6 +47,7 @@ public:
       // go up one more time
       ptr_ = ptr_->parent;
     }
+    // when we return, ptr_ should point to the destination node.
     return *this;
   }
   tree_iterator<T> operator++(int) {  tree_iterator<T> temp(*this);  ++(*this);  return temp;  }
diff --git a/lectures/19_trees_II/ds_set_main.cpp b/lectures/19_trees_II/ds_set_main.cpp
index 4e76ef0..8f35199 100644
--- a/lectures/19_trees_II/ds_set_main.cpp
+++ b/lectures/19_trees_II/ds_set_main.cpp
@@ -8,8 +8,11 @@ int main() {
     // insert some values into the set
     numbers.insert(10);
     numbers.insert(5);
+    numbers.insert(88);
     numbers.insert(20);
+    numbers.insert(49);
     numbers.insert(15);
+    numbers.insert(36);
     numbers.insert(5); // Duplicate value (won't be inserted)
 
     // print the elements of the set
@@ -27,5 +30,13 @@ int main() {
         std::cout << value << " is not found in the set." << std::endl;
     }
 
+    // check if a specific value exists in the set
+    value = 66;
+    if (numbers.find(value) != numbers.end()) {
+        std::cout << value << " is found in the set." << std::endl;
+    } else {
+        std::cout << value << " is not found in the set." << std::endl;
+    }
+
     return 0;
 }
diff --git a/lectures/20_trees_III/ds_set_main.cpp b/lectures/20_trees_III/ds_set_main.cpp
new file mode 100644
index 0000000..2885b44
--- /dev/null
+++ b/lectures/20_trees_III/ds_set_main.cpp
@@ -0,0 +1,41 @@
+#include <iostream>
+#include "ds_set_ptrs.h"
+
+int main() {
+    // create a set of integers
+    ds_set<int> numbers;
+
+    // insert some values into the set
+    numbers.insert(10);
+    numbers.insert(5);
+    numbers.insert(88);
+    numbers.insert(20);
+    numbers.insert(49);
+    numbers.insert(15);
+    numbers.insert(36);
+    numbers.insert(5); // Duplicate value (won't be inserted)
+
+    // print the elements of the set
+    std::cout << "The elements in the set are:" << std::endl;
+    for(ds_set<int>::iterator itr = numbers.begin(); itr != numbers.end(); ++itr){
+        std::cout << *itr << " ";
+    }
+    std::cout << std::endl;
+
+    // check if a specific value exists in the set
+    int value = 15;
+    if (numbers.find(value) != numbers.end()) {
+        std::cout << value << " is found in the set." << std::endl;
+    } else {
+        std::cout << value << " is not found in the set." << std::endl;
+    }
+
+    // check if a specific value exists in the set
+    value = 66;
+    if (numbers.find(value) != numbers.end()) {
+        std::cout << value << " is found in the set." << std::endl;
+    } else {
+        std::cout << value << " is not found in the set." << std::endl;
+    }
+    return 0;
+}
diff --git a/lectures/20_trees_III/ds_set_ptrs.h b/lectures/20_trees_III/ds_set_ptrs.h
new file mode 100644
index 0000000..bc5d868
--- /dev/null
+++ b/lectures/20_trees_III/ds_set_ptrs.h
@@ -0,0 +1,166 @@
+#include <utility> // for std::pair
+#include <vector>
+
+template <class T>
+class TreeNode {
+public:
+	T key;
+	TreeNode* left;
+	TreeNode* right;
+	// one way to allow implementation of iterator increment & decrement
+	TreeNode* parent;
+	// constructor
+	TreeNode(const T& k) {
+		key = k; 
+		left = NULL;
+		right = NULL;
+	}
+};
+
+// -------------------------------------------------------------------
+// TREE NODE ITERATOR CLASS
+template <class T>
+class tree_iterator {
+public:
+	// default constructor
+	tree_iterator() = default; // including = default explicitly tells someone reading the code that 
+				   // you intend for the compiler to generate the default constructor. 
+				   // It makes your intent clearer.
+	// constructor
+	tree_iterator(std::vector<TreeNode<T>*> p) : ptrs(p) {}
+	// copy constructor
+	tree_iterator(const tree_iterator& other) : ptrs(other.ptrs) {}
+	// destructor
+	~tree_iterator() {}
+	// assignment operator
+	tree_iterator& operator=(const tree_iterator& other) { ptrs = other.ptrs;  return *this; }
+	// operator* gives constant access to the value at the pointer
+	const T& operator*() const { return (ptrs.back())->key; }
+	// comparison operators are straightforward
+	bool operator== (const tree_iterator& other) { return ptrs == other.ptrs; }
+	bool operator!= (const tree_iterator& other) { return ptrs != other.ptrs; }
+	// increment & decrement operators
+	tree_iterator<T> & operator++() { 
+		// prefix ++
+		if (ptrs.empty()) return *this; // end() reached
+		// ptrs.back() gives us the pointer points to the current node, i.e, that's where we are now.
+		TreeNode<T>* curr = ptrs.back();
+		// case 1: has right subtree
+		if (curr->right) {
+			curr = curr->right;
+			ptrs.push_back(curr);
+			// go as left as possible
+			while (curr->left) {
+				curr = curr->left;
+				ptrs.push_back(curr);
+			}
+		}
+		// case 2: no right subtree, go up
+		else {
+			TreeNode<T>* last = ptrs.back();
+			ptrs.pop_back();
+			// keep going up as long as I am (here I means last) the right child of my parent.
+			while (!ptrs.empty() && ptrs.back()->right == last) {
+				last = ptrs.back();
+				ptrs.pop_back();
+			}
+			// if ptrs empty now, we hit end()
+		}
+		// when we return, ptrs.back() should point to the destination node.
+		return *this;
+	}
+	tree_iterator<T> operator++(int) {  tree_iterator<T> temp(*this);  ++(*this);  return temp;  }
+	tree_iterator<T> & operator--() { /* implementation omitted */ }
+	tree_iterator<T> operator--(int) {  tree_iterator<T> temp(*this);  --(*this);  return temp;  }
+private:
+	// representation
+	std::vector<TreeNode<T>*> ptrs;  // store pointers to every node which is on the path from root to current node
+};
+
+// internally it's a binary search tree
+template <class T>
+class ds_set {
+public:
+	ds_set(){
+		root = NULL;
+		m_size = 0;
+	}
+	typedef tree_iterator<T> iterator;
+	int size() { return m_size; }
+	iterator find(const T& key){
+		std::vector<TreeNode<T>*> ptrs;
+		return find(key, root, ptrs);
+	}
+	std::pair<iterator, bool> insert(const T& key){ 
+		std::pair<iterator, bool> temp;
+		std::vector<TreeNode<T>*> ptrs;
+		temp = insertHelper(key, root, ptrs);
+	        if(temp.second	== true){
+			++m_size;
+		}
+		return temp;
+	}
+	// ITERATORS
+	// return an iterator to the first (leftmost) node of the binary search tree, 
+	// which can be found by traversing to the leftmost node starting from the root.
+	tree_iterator<T> begin() const {
+		std::vector<TreeNode<T>*> ptrs;
+		TreeNode<T>* p = root;
+		while (p) {
+			ptrs.push_back(p);
+			p = p->left;  // go left
+		}
+		return tree_iterator<T>(ptrs);
+	}
+	tree_iterator<T> end() const { return tree_iterator<T>(); }
+private: 
+	TreeNode<T>* root;
+	int m_size;
+	iterator find(const T& key, TreeNode<T>* root, std::vector<TreeNode<T>*>& ptrs);
+	// as there are multiple templated classes involved, writing this function outside of the class definition may be too complicated.
+	// helper function to perform the insertion
+	std::pair<iterator, bool> insertHelper(const T& key, TreeNode<T>*& node, std::vector<TreeNode<T>*>& ptrs) {
+		// if node is nullptr, insert the key here
+		if (!node) {
+			node = new TreeNode<T>(key);
+			ptrs.push_back(node);  // add the new node to the path
+			return {iterator(ptrs), true};  // return an iterator to the new node, and true for success
+		}
+
+		// if key is already present in the tree, no insertion occurs
+		if (key == node->key) {
+			return {iterator(ptrs), false};  // return an iterator to the existing node, and false for failure
+		}
+
+		// add the current node to the path
+		ptrs.push_back(node);
+
+		// recursively insert into the left or right subtree
+		if (key < node->key) {
+			return insertHelper(key, node->left, ptrs);  // Traverse left
+		} else {
+			return insertHelper(key, node->right, ptrs);  // Traverse right
+		}
+	}
+
+};
+
+template <class T>
+typename ds_set<T>::iterator ds_set<T>::find(const T& key, TreeNode<T>* root, std::vector<TreeNode<T>*>& ptrs){
+	// base case (if root doesn't even exist)
+	if(root == NULL){
+		return end();
+	}
+	
+	 // add current node to the path
+        ptrs.push_back(root);
+	// general case
+	if(key < root->key){
+		return find(key, root->left, ptrs);
+	}else if(key > root->key){
+		return find(key, root->right, ptrs);
+	}else{
+		return tree_iterator(ptrs);
+	}
+}
+