Changed StackList.__init__

Aufgabe03a.py

-class Stack(list):
+class Stack():
+
     def __init__(self, l=[]):
         self.stack = l
-    
+
     def __str__(self):
         max_length = len(str(max(self.stack, key=lambda x: len(str(x)))))
         result = ""
@@ -12,7 +13,7 @@ class Stack(list):
 
     def push(self, element):
         self.stack.append(element)
-    
+
     def pop(self):
         element = self.stack[-1]
         del self.stack[-1]
@@ -34,45 +35,48 @@ class ListElement:
         self.next = next
 
 class StackList:
-        
-    def __init__(self, l):
-        self.first_element = ListElement(l[-1])
-        element = self.first_element
-        for value in l[1::-1]:
-            while element.next is not None:
-                element = element.next
-            element.next = ListElement(value)
-    
+
+    def __init__(self, l=None):
+        if l is None:
+            self.first_element = None
+        else:
+            self.first_element = ListElement(l[-1])
+            element = self.first_element
+            for value in l[1::-1]:
+                while element.next is not None:
+                    element = element.next
+                element.next = ListElement(value)
+
     def __str__(self):
         return_string = ""
         element = self.first_element
         while element.next is not None:
             return_string += str(element.value) + " -> "
             element = element.next
-        
+
         return return_string + str(element.value)
 
     def push(self, value):
         self.first_element = ListElement(value, self.first_element)
-    
+
     def pop(self):
         value = self.first_element.value
         self.first_element = self.first_element.next
         return value
-    
+
     def top(self):
         return self.first_element.value
-    
+
     def size(self):
         if self.first_element is None:
             return 0
-        
+
         n = 1
         element = self.first_element
         while element.next is not None:
             n += 1
             element = element.next
-        
+
         return n
 
     def isEmpty(self):