reverse liststack direction

src/stack.rs

@@ -40,14 +40,18 @@ impl Stack for ListStack {
 
     fn push_val(&mut self, i: i32) {
         match self {
-            Val(v, o) => *self = Val(i, Some(Box::new(Val(*v,o.take())))),
-            Nil => *self = Val(i,None),
+            Val(value, other) => {
+                // create a new smart pointer to the current node
+                let new_boxed = Box::new(Val(*value, other.take()));
+                *self = Val(i, Some(new_boxed));
+            },
+            Nil => *self = Val(i, None) 
         };
     }
 
     fn top_val(&self) -> Option<&i32> {
         match self {
-            Val(val,_o) => Some(val),
+            Val(value, _) => Some(value),
             Nil => None
         }
     }
@@ -55,10 +59,12 @@ impl Stack for ListStack {
     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