pass tests

src/stack.rs

 use crate::Stack;
 
-// TODO Complete implementation
 impl Stack for Vec<i32> {
     fn init() -> Self {
-        return vec![]
+        Vec::new()
     }
 
     fn push_val(&mut self, i: i32) {
-        self.push(i)
+        self.push(i);
     }
 
     fn top_val(&self) -> Option<&i32> {
-        return self.last()
+        self.last()
     }
 
     fn pop_val(&mut self) -> Option<i32> {
-        return self.pop()
+        self.pop()
     }
 
     fn is_empty(&self) -> bool {
-        return self.is_empty()
+        self.is_empty()
     }
 }
 
+// Nil is only used for truly empty lists, nonempty lists end in Val(x, None)
 #[derive(Debug)]
 pub enum ListStack {
     Val(i32, Option<Box<ListStack>>),
@@ -48,28 +48,27 @@ impl Stack for ListStack {
     fn top_val(&self) -> Option<&i32> {
         match self {
             Val(val,_o) => Some(val),
-            Nil => None,
+            Nil => None
         }
     }
 
     fn pop_val(&mut self) -> Option<i32> {
         match self {
             Val(value, other) => {
-                let popped_value = *value;
                 match other.take() {
                     None => *self = Nil,
                     Some(other) => *self = *other,
                 };
                 Some(popped_value)
             }
-            Nil => None,
+            Nil => None
         }
     }
 
     fn is_empty(&self) -> bool {
         match self {
-            Val(_v,_o) => false,
             Nil => true,
+            _ => false
         }
     }
 }

src/syntax_tree.rs

@@ -23,12 +23,12 @@ fn next_id() -> ID {
 impl<T> SyntaxTree<T> {
     /// Create a SyntaxTree with a root node that carries the given value
     pub fn new(value: T) -> SyntaxTree<T> {
-        todo!()
+        SyntaxTree { id: next_id(), value, children: Vec::new() }
     }
 
     /// Add another SyntaxTree as last child of this tree
     pub fn push_node(&mut self, new_node: SyntaxTree<T>) {
-        todo!()
+        self.children.push(new_node);
     }
 
     /// Create a new SyntaxTree with a root node that carries the given value. Add the created tree
@@ -39,7 +39,7 @@ impl<T> SyntaxTree<T> {
 
     /// Add another SyntaxTree as first child of this tree
     pub fn prepend_node(&mut self, new_node: SyntaxTree<T>) {
-        todo!()
+        self.children.insert(0, new_node);
     }
 
     /// Create a new SyntaxTree with a root node that carries the given value. Add the created tree
@@ -66,7 +66,15 @@ impl<T> SyntaxTree<T> {
         if predicate(self) {
             Some(self)
         } else {
-            todo!()
+            for child in self.children.iter() {
+                match child.find_node(predicate) {
+                    None => continue,
+                    some => return some
+                }
+            }
+
+            // nothing was found in any of the children
+            None
         }
     }
 
@@ -77,7 +85,7 @@ impl<T> SyntaxTree<T> {
         &mut self,
         predicate: fn(&SyntaxTree<T>) -> bool,
     ) -> Option<&SyntaxTree<T>> {
-        todo!()
+        self.find_node(predicate)
     }
 
     /// Return a reference to the value carried by the root of this tree