diff --git a/week3/linked_list/src/linked_list.rs b/week3/linked_list/src/linked_list.rs
index 7527a1d..61b0215 100644
--- a/week3/linked_list/src/linked_list.rs
+++ b/week3/linked_list/src/linked_list.rs
@@ -1,11 +1,13 @@
 use std::fmt;
 use std::option::Option;
 
+#[derive(Debug)]
 pub struct LinkedList<T> {
     head: Option<Box<Node<T>>>,
     size: usize,
 }
 
+#[derive(Debug)]
 struct Node<T> {
     value: T,
     next: Option<Box<Node<T>>>,
@@ -43,7 +45,9 @@ impl<T> LinkedList<T> {
         Some(node.value)
     }
 
-
+    pub fn iter<'a>(&'a self) -> LinkedListIter<'a, T> {
+        LinkedListIter { item: &self.head }
+    }
 }
 
 
@@ -110,3 +114,56 @@ impl<T:PartialEq> PartialEq for LinkedList<T> {
     }
 }
 
+pub struct LinkedListIter<'a, T> {
+    item: &'a Option<Box<Node<T>>>,
+}
+
+impl<'a, T> Iterator for LinkedListIter<'a, T> {
+    type Item = &'a T;
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.item {
+            Some(node) => {
+                self.item = &node.next;
+                Some(&node.value)
+            },
+            None => None,
+        }
+    }
+}
+
+impl<'a, T> IntoIterator for &'a LinkedList<T> {
+    type Item = &'a T;
+    type IntoIter = LinkedListIter<'a, T>;
+    fn into_iter(self) -> Self::IntoIter {
+        LinkedListIter {item: &self.head}
+    }
+}
+
+pub struct IntoIter<T>(LinkedList<T>);
+
+impl<T> Iterator for IntoIter<T> {
+    type Item = T;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.pop_front()
+    }
+}
+
+impl<T> IntoIterator for LinkedList<T> {
+    type Item = T;
+    type IntoIter = IntoIter<T>;
+    fn into_iter(self) -> Self::IntoIter {
+        IntoIter(self)
+    }
+}
+
+pub trait ComputeNorm {
+    fn compute_norm(&self) -> f64 {
+        0.0
+    }
+}
+
+impl ComputeNorm for LinkedList<f64> {
+    fn compute_norm(&self) -> f64 {
+        self.iter().map(|x| {x * x}).sum::<f64>().sqrt()
+    }
+}
\ No newline at end of file
diff --git a/week3/linked_list/src/main.rs b/week3/linked_list/src/main.rs
index 981d23e..e8e6de3 100644
--- a/week3/linked_list/src/main.rs
+++ b/week3/linked_list/src/main.rs
@@ -1,12 +1,14 @@
 use linked_list::LinkedList;
+
+use crate::linked_list::ComputeNorm;
 pub mod linked_list;
 
 fn main() {
-    let mut list: LinkedList<u32> = LinkedList::new();
+    let mut list: LinkedList<String> = LinkedList::new();
     assert!(list.is_empty());
     assert_eq!(list.get_size(), 0);
     for i in 1..12 {
-        list.push_front(i);
+        list.push_front(i.to_string());
     }
     println!("{}", list);
     println!("list size: {}", list.get_size());
@@ -20,9 +22,24 @@ fn main() {
     list2.pop_front();
     println!("list2.pop_front: {}", list2);
     println!("list == list2 ?= {}", list == list2);
-    
+
     // If you implement iterator trait:
-    //for val in &list {
-    //    println!("{}", val);
-    //}
+    for val in &list {
+        // reference iterator
+        print!("{} ", val);
+    }
+    println!();
+    for val in list {
+        // consuming iterator
+        print!("{} ", val);
+    }
+    println!();
+    // let mut error = list.into_iter();
+    // list 's moved in the second loop, so it cannot be used again
+    // assert_eq!(Some(String::from("10")), error.next());
+    let mut list: LinkedList<f64> = LinkedList::new();
+    for i in 1..12 {
+        list.push_front(f64::from(i));
+    }
+    println!("ComputeNorm for {:?} = {}", list, list.compute_norm());
 }