diff --git a/lectures/17_trees_I/ds_set_lec17.h b/lectures/17_trees_I/ds_set_lec17.h
new file mode 100644
index 0000000..66a27f8
--- /dev/null
+++ b/lectures/17_trees_I/ds_set_lec17.h
@@ -0,0 +1,183 @@
+// Partial implementation of binary-tree based set class similar to std::set.  
+// The iterator increment & decrement operations have been omitted.
+#ifndef ds_set_h_
+#define ds_set_h_
+#include <iostream>
+#include <utility>
+
+// -------------------------------------------------------------------
+// TREE NODE CLASS 
+template <class T>
+class TreeNode {
+public:
+  TreeNode() : left(NULL), right(NULL) {}
+  TreeNode(const T& init) : value(init), left(NULL), right(NULL) {}
+  T value;
+  TreeNode* left;
+  TreeNode* right;
+};
+
+// -------------------------------------------------------------------
+// TREE NODE ITERATOR CLASS
+template <class T>
+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() {}
+  tree_iterator& operator=(const tree_iterator& old) { ptr_ = old.ptr_;  return *this; }
+
+  // operator* gives constant access to the value at the pointer
+  const T& operator*() const { return ptr_->value; }
+  // comparions operators are straightforward
+  bool operator== (const tree_iterator& rgt) { return ptr_ == rgt.ptr_; }
+  bool operator!= (const tree_iterator& rgt) { return ptr_ != rgt.ptr_; }
+
+private:
+  // representation
+  TreeNode<T>* ptr_;
+};
+
+// -------------------------------------------------------------------
+// DS_SET CLASS
+template <class T>
+class ds_set {
+public:
+  ds_set() : root_(NULL), size_(0) {}
+  ds_set(const ds_set<T>& old) : size_(old.size_) { 
+    root_ = this->copy_tree(old.root_); }
+  ~ds_set() {
+    this->destroy_tree(root_);
+    root_ = NULL;
+  }
+  ds_set& operator=(const ds_set<T>& old) {
+    if (&old != this) {
+      this->destroy_tree(root_);
+      root_ = this->copy_tree(old.root_);
+      size_ = old.size_;
+    }
+    return *this;
+  }
+
+  typedef tree_iterator<T> iterator;
+
+  int size() const { return size_; }
+  bool operator==(const ds_set<T>& old) const { return (old.root_ == this->root_); }
+
+  // FIND, INSERT & ERASE
+  iterator find(const T& key_value) { return find(key_value, root_); }
+  std::pair< iterator, bool > insert(T const& key_value) { return insert(key_value, root_); }
+  int erase(T const& key_value) { return erase(key_value, root_); }
+  
+  // OUTPUT & PRINTING
+  friend std::ostream& operator<< (std::ostream& ostr, const ds_set<T>& s) {
+    s.print_in_order(ostr, s.root_);
+    return ostr;
+  }
+  void print_as_sideways_tree(std::ostream& ostr) const {
+    print_as_sideways_tree(ostr, root_, 0);
+  }
+
+  // ITERATORS
+  iterator begin() const { 
+    if (!root_) return iterator(NULL);
+    TreeNode<T>* p = root_;
+    while (p->left) p = p->left;
+    return iterator(p);
+  }
+  iterator end() const { return iterator(NULL); }
+
+private:
+  // REPRESENTATION
+  TreeNode<T>* root_;
+  int size_;
+
+  // PRIVATE HELPER FUNCTIONS
+  TreeNode<T>*  copy_tree(TreeNode<T>* old_root) {
+    if (old_root == NULL) 
+      return NULL;
+    TreeNode<T> *answer = new TreeNode<T>();
+    answer->value = old_root->value;
+    answer->left = copy_tree(old_root->left);
+    answer->right = copy_tree(old_root->right);
+    return answer;
+  }
+
+  void destroy_tree(TreeNode<T>* p) {
+    if (!p) return;
+    destroy_tree(p->right);
+    destroy_tree(p->left);
+    delete p;
+  }
+
+  iterator find(const T& key_value, TreeNode<T>* p) {
+    if (!p) return iterator(NULL);
+    if (p->value > key_value)
+      return find(key_value, p->left);
+    else if (p->value < key_value)
+      return find(key_value, p->right);
+    else
+      return iterator(p);
+  }
+
+  std::pair<iterator,bool> insert(const T& key_value, TreeNode<T>*& p) {
+    if (!p) {
+      p = new TreeNode<T>(key_value);
+      this->size_++;
+      return std::pair<iterator,bool>(iterator(p), true);
+    }
+    else if (key_value < p->value)
+      return insert(key_value, p->left);
+    else if (key_value > p->value)
+      return insert(key_value, p->right);
+    else
+      return std::pair<iterator,bool>(iterator(p), false);
+  }
+  
+  int erase(T const& key_value, TreeNode<T>* &p) {
+    if (!p) return 0;
+ 
+    // look left & right
+   if (p->value < key_value)
+      return erase(key_value, p->right);
+    else if (p->value > key_value)
+      return erase(key_value, p->left);
+
+    // Found the node.  Let's delete it
+    assert (p->value == key_value);
+    if (!p->left && !p->right) { // leaf
+      delete p; p=NULL; 
+    } else if (!p->left) { // no left child
+      TreeNode<T>* q = p; p=p->right; delete q; 
+    } else if (!p->right) { // no right child
+      TreeNode<T>* q = p; p=p->left; delete q; 
+    } else { // Find rightmost node in left subtree
+      TreeNode<T>* &q = p->left;
+      while (q->right) q = q->right;
+      p->value = q->value;
+      int check = erase(q->value, q);
+      assert (check == 1);
+    }
+    return 1;
+  }
+
+  void print_in_order(std::ostream& ostr, const TreeNode<T>* p) const {
+    if (p) {
+      print_in_order(ostr, p->left);
+      ostr << p->value << "\n";
+      print_in_order(ostr, p->right);
+    }
+  }
+
+  void print_as_sideways_tree(std::ostream& ostr, const TreeNode<T>* p, int depth) const {
+    if (p) {
+      print_as_sideways_tree(ostr, p->right, depth+1);
+      for (int i=0; i<depth; ++i) ostr << "    ";
+      ostr << p->value << "\n";
+      print_as_sideways_tree(ostr, p->left, depth+1);
+    }
+  }
+};
+
+#endif