diff --git a/src/hands_on.rs b/src/hands_on.rs
new file mode 100644
index 0000000..d5af167
--- /dev/null
+++ b/src/hands_on.rs
@@ -0,0 +1,3 @@
+pub mod tree_traversals;
+
+pub use crate::hands_on::tree_traversals::*;
\ No newline at end of file
diff --git a/src/hands_on/tree_traversals.rs b/src/hands_on/tree_traversals.rs
new file mode 100644
index 0000000..774ad3b
--- /dev/null
+++ b/src/hands_on/tree_traversals.rs
@@ -0,0 +1,243 @@
+// basic bin tree implementation (on array)
+struct Node {
+    key: u32,
+    id_left: Option<usize>,
+    id_right: Option<usize>,
+}
+
+impl Node {
+    fn new(key: u32) -> Self {
+        Self {
+            key, 
+            id_left: None,
+            id_right: None,
+        }
+    }
+}
+
+struct Tree {
+    nodes: Vec<Node>
+}
+
+impl Tree {
+    pub fn with_root(key: u32) -> Self {
+        Self {
+            nodes: vec![Node::new(key)],
+        }
+    }
+
+    /// Adds a child to the node with `parent_id` and returns the id of the new node. 
+    /// The new node has the specified `key`. The new node is the left  child of the  
+    /// node `parent_id` iff `is_left` is `true`, the right child otherwise.
+    ///
+    /// # Panics
+    /// Panics if the `parent_id` does not exist, or if the node `parent_id ` has  
+    /// the child already set.
+    pub fn add_node(&mut self, parent_id: usize, key: u32, is_left: bool) -> usize {
+        assert!(
+            parent_id < self.nodes.len(),
+            "Parent node id does not exist"
+        );
+        if is_left {
+            assert!(
+                self.nodes[parent_id].id_left == None,
+                "Parent node has the left child already set"
+            );
+        } else {
+            assert!(
+                self.nodes[parent_id].id_right == None,
+                "Parent node has the right child already set"
+            );
+        }
+
+        // child position in array
+        let child_id = self.nodes.len();
+        self.nodes.push(Node::new(key));
+
+        let child = if is_left {
+            &mut self.nodes[parent_id].id_left
+        } else {
+            &mut self.nodes[parent_id].id_right
+        };
+
+        // parent.id_left or parent.id_right = child_id
+        *child = Some(child_id);
+
+        child_id
+    }
+
+    /// Returns the sum of all the keys in the tree
+    pub fn sum(&self) -> u32 {
+        self.rec_sum(Some(0))
+    }
+
+    /// A private recursive function that computes the sum of 
+    /// nodes in the subtree rooted at `node_id`
+    fn rec_sum(&self, node_id: Option<usize>) -> u32 {
+        if let Some(id) = node_id {
+            assert!(id < self.nodes.len(), "Node id is out of range");
+            let node = &self.nodes[id];
+
+            let left_sum = self.rec_sum(node.id_left);
+            let right_sum = self.rec_sum(node.id_right);
+
+            return left_sum + right_sum + node.key;
+        }
+
+        0
+    }
+
+    /// A method to check if the binary tree is a Binary Search Tree
+    pub fn is_bst(&self) -> bool {
+        self.rec_is_bst(Some(0))
+    }
+    
+    fn rec_is_bst(&self, node_id: Option<usize>) -> bool {
+        if let Some(id) = node_id {
+            assert!(id < self.nodes.len(), "Node id is out of range");
+            let node = &self.nodes[id];
+
+            if let Some(id_left) = node.id_left {
+                let left_node = &self.nodes[id_left];
+
+                if left_node.key > node.key {
+                    return false;
+                }
+            }
+
+            if let Some(id_right) = node.id_right {
+                let right_node = &self.nodes[id_right];
+
+                if right_node.key <= node.key {
+                    return false;
+                }
+            }
+
+            return self.rec_is_bst(node.id_left) && self.rec_is_bst(node.id_right);   
+        }
+
+        true
+    }
+
+    pub fn is_balanced(&self) -> bool {
+        let mut height = 0;
+        self.rec_is_balanced(Some(0), &mut height)
+    }
+
+    fn rec_is_balanced(&self, node_id: Option<usize>, height: &mut i32) -> bool {
+        if let Some(id) = node_id {
+            assert!(id < self.nodes.len(), "Node id is out of range");
+            let node = &self.nodes[id];
+
+            let mut left_height = 0;
+            let mut right_height = 0;
+            
+            let left_ok = if node.id_left == None { true } else { 
+                self.rec_is_balanced(node.id_left, &mut left_height)
+            };
+
+            let right_ok = if node.id_right == None { true } else { 
+                self.rec_is_balanced(node.id_right, &mut right_height)
+            };
+
+            // the height of the current node is the maximum height between left and right
+            // plus the current node itself
+            *height = left_height.max(right_height) + 1;
+
+            if (left_height - right_height).abs() > 1 {
+                return  false;
+            }
+
+            return left_ok && right_ok;
+        }
+
+        true
+    }
+
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_sum() {
+        let mut tree = Tree::with_root(10);
+
+        assert_eq!(tree.sum(), 10);
+
+        tree.add_node(0, 5, true); // id 1
+        tree.add_node(0, 22, false); // id 2
+
+        assert_eq!(tree.sum(), 37);
+
+        tree.add_node(1, 7, false); // id 3
+        tree.add_node(2, 20, true); // id 4
+
+        assert_eq!(tree.sum(), 64);
+    }
+
+    #[test]
+    fn test_is_bst() {
+        let mut tree = Tree::with_root(5);
+
+        tree.add_node(0, 3, true);
+        tree.add_node(0, 7, false);
+
+        assert_eq!(tree.is_bst(), true);
+
+        tree.add_node(1, 1, true);
+        tree.add_node(1, 4, false);
+
+        assert_eq!(tree.is_bst(), true);
+
+        tree.add_node(2, 6, true);
+        tree.add_node(2, 8, false);
+
+        assert_eq!(tree.is_bst(), true);
+
+        tree.add_node(6, 6, false);
+
+        assert_eq!(tree.is_bst(), false);
+    }
+
+    #[test]
+    fn test_is_balanced() {
+        let mut tree = Tree::with_root(1);
+
+        assert_eq!(tree.is_balanced(), true);
+
+        tree.add_node(0, 2, true); // id 1
+
+        assert_eq!(tree.is_balanced(), true);
+
+        tree.add_node(1, 3, true); // id 2
+
+        assert_eq!(tree.is_balanced(), false);
+
+        tree.add_node(2, 6, true); // id 3
+
+        assert_eq!(tree.is_balanced(), false);
+
+        tree.add_node(0, 4, false); // id 4
+
+        assert_eq!(tree.is_balanced(), false);
+
+        tree.add_node(4, 5, false); // id 5
+
+        assert_eq!(tree.is_balanced(), false);
+
+        tree.add_node(5, 7, false); // id 6
+
+        assert_eq!(tree.is_balanced(), false);
+
+        tree.add_node(1, 8, false); // id 7 
+
+        assert_eq!(tree.is_balanced(), false);
+
+        tree.add_node(4, 9, true); // id 8
+
+        assert_eq!(tree.is_balanced(), true);
+
+    }
+}
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index 249810d..c0b6a1a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,6 +2,7 @@ use crate::problems::replace_elements;
 
 pub mod basics;
 pub mod problems;
+pub mod hands_on;
 
 fn main() {
     let arr = vec![17,18,5,4,6,1];
diff --git a/src/problems/lec4.rs b/src/problems/lec4.rs
index ca8f12e..aa67501 100644
--- a/src/problems/lec4.rs
+++ b/src/problems/lec4.rs
@@ -41,3 +41,36 @@ pub fn max_sum_helper(root: &Option<Rc<RefCell<TreeNode>>>, acc: &mut i32) -> i3
     }
 }
 
+pub fn is_balanced(root: Option<Rc<RefCell<TreeNode>>>) -> bool {
+    let mut height = 0;
+    rec_is_balanced(&root, &mut height)
+}
+
+fn rec_is_balanced(root: &Option<Rc<RefCell<TreeNode>>>, height: &mut i32) -> bool {
+    match root {
+        None => true,
+        Some(tree_node) => {
+            let root_val = tree_node.as_ref().borrow();
+            let mut left_height = 0;
+            let mut right_height = 0;
+            let mut left_ok = true;
+            let mut right_ok = true;
+
+            if let Some(_) = root_val.left {
+                left_ok = rec_is_balanced(&root_val.left, &mut left_height);
+            }
+
+            if let Some(_) = root_val.right {
+                right_ok = rec_is_balanced(&root_val.right, &mut right_height);
+            }
+
+            *height = left_height.max(right_height) + 1;
+
+            if (left_height - right_height).abs() > 1 {
+                return false;
+            }
+
+            left_ok && right_ok
+        }
+    }
+}