diff --git a/book/book_second_edition.pdf b/book/book_second_edition.pdf
index 4495060..3c7084e 100644
Binary files a/book/book_second_edition.pdf and b/book/book_second_edition.pdf differ
diff --git a/src/abstract_structures/queues/deque.py b/src/abstract_structures/queues/deque.py
index 86573bd..a6bb95a 100644
--- a/src/abstract_structures/queues/deque.py
+++ b/src/abstract_structures/queues/deque.py
@@ -1,46 +1,40 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
+#!/usr/bin/python
 
+__author__ = "Mari Wahl"
+__email__ = "marina.w4hl@gmail.com"
 
+''' a class for a double ended queue (also inefficient) '''
 
-class Deque(object):
-    ''' a class for a double ended queue '''
-    def __init__(self):
-        self.items = []
+from queue import Queue
 
-    def is_empty(self):
-        return self.items == []
+class Deque(Queue):
 
-    def add_front(self, item):
+    def enqueue_back(self, item):
         self.items.append(item)
 
-    def add_rear(self, item):
-        self.items.insert(0, item)
-
-    def remove_front(self):
-        return self.items.pop()
-
-    def remove_rear(self):
+    def dequeue_front(self):
         return self.items.pop(0)
 
-    def size(self):
-        return len(self.items)
-        
-    def __repr__(self):
-        return '{}'.format(self.items)
-
-
-def main():
-    dq = Deque()
-    dq.add_front(1)
-    dq.add_front(2)
-    dq.add_front(3)
-    dq.add_rear(40)
-    dq.add_rear(50)
-    print(dq.size())  
-    print(dq)  
 
 
 if __name__ == '__main__':
-    main()    
+    queue = Deque()
+    print("Is the queue empty? ", queue.isEmpty())
+    print("Adding 0 to 10 in the queue...")
+    for i in range(10):
+        queue.enqueue(i)
+    print("Queue size: ", queue.size())
+    print("Queue peek : ", queue.peek())
+    print("Dequeue...", queue.dequeue())
+    print("Queue peek: ", queue.peek())
+    print("Is the queue empty? ", queue.isEmpty())
+    print(queue)
+
+    print("\nNow using the dequeue methods...")
+    print("Dequeue from front...", queue.dequeue_front())
+    print("Queue peek: ", queue.peek())
+    print(queue)
+    print("Queue from back...")
+    queue.enqueue_back(50)
+    print("Queue peek: ", queue.peek())
+    print(queue)
diff --git a/src/abstract_structures/queues/linked_queue.py b/src/abstract_structures/queues/linked_queue.py
index 4aadc0a..c25dad5 100644
--- a/src/abstract_structures/queues/linked_queue.py
+++ b/src/abstract_structures/queues/linked_queue.py
@@ -1,61 +1,81 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
+#!/usr/bin/python
+
+__author__ = "Mari Wahl"
+__email__ = "marina.w4hl@gmail.com"
+
+''' Queue acts as a container for nodes (objects) that are inserted and removed according FIFO'''
+
 
 class Node(object):
-    def __init__(self, value):
+    def __init__(self, value=None, pointer=None):
         self.value = value
-        self.next = None
-        
+        self.pointer = None
+
 
 class LinkedQueue(object):
-    ''' Queue acts as a container for nodes (objects) that are inserted and removed according FIFO'''
     def __init__(self):
-        self.front = None
-        self.back = None
+        self.head = None
+        self.tail = None
+
 
     def isEmpty(self):
-        return bool(self.front)
+        return bool(self.head)
+
 
     def dequeue(self):
-        if self.front:
-            value = self.front.value
-            self.front = self.front.next
+        if self.head:
+            value = self.head.value
+            self.head = self.head.pointer
             return value
-        raise Exception('Queue is empty, cannot dequeue.')
+        else:
+            print('Queue is empty, cannot dequeue.')
+
 
     def enqueue(self, value):
         node = Node(value)
-        if self.front:
-            self.back.next = node
-            self.back = node
+        if not self.head:
+            self.head = node
+            self.tail = node
         else:
-            self.front = node
-            self.back = node
-        return True
+            if self.tail:
+                self.tail.pointer = node
+            self.tail = node
+
 
     def size(self):
-        node = self.front
-        num_nodes = 1
-        while node.next:
-            num_nodes += 1
-            node = node.next
+        node = self.head
+        num_nodes = 0
+        while node:
+                num_nodes += 1
+                node = node.pointer
         return num_nodes
 
+
     def peek(self):
-        return self.front.value
+        return self.head.value
+
+
+    def _print(self):
+        node = self.head
+        while node:
+            print(node.value)
+            node = node.pointer
 
 
-def main():
-    queue = LinkedQueue()
-    queue.enqueue(1)
-    queue.enqueue(2)
-    queue.enqueue(3)
-    print(queue.size())    
-    print(queue.peek())   
-    print(queue.dequeue())  
-    print(queue.peek())    
 
 
 if __name__ == '__main__':
-    main()      
+    queue = LinkedQueue()
+    print("Is the queue empty? ", queue.isEmpty())
+    print("Adding 0 to 10 in the queue...")
+    for i in range(10):
+        queue.enqueue(i)
+    print("Is the queue empty? ", queue.isEmpty())
+    queue._print()
+
+    print("Queue size: ", queue.size())
+    print("Queue peek : ", queue.peek())
+    print("Dequeue...", queue.dequeue())
+    print("Queue peek: ", queue.peek())
+    queue._print()
+
diff --git a/src/abstract_structures/queues/palindrome_checker_with_deque.py b/src/abstract_structures/queues/palindrome_checker_with_deque.py
index 9591099..ed8f591 100644
--- a/src/abstract_structures/queues/palindrome_checker_with_deque.py
+++ b/src/abstract_structures/queues/palindrome_checker_with_deque.py
@@ -1,34 +1,45 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
+#!/usr/bin/python
+
+__author__ = "Mari Wahl"
+__email__ = "marina.w4hl@gmail.com"
 
-import sys
 import string
-import collections  
+import collections
 
+from deque import Deque
+
+STRIP = string.whitespace + string.punctuation + "\"'"
+
+
+""" Using our deque class and Python's deque class """
 def palindrome_checker_with_deque(str1):
-    d = collections.deque()
-    eq = True
-    strip = string.whitespace + string.punctuation + "\"'" 
-    for s in str1.lower():
-        if s not in strip: d.append(s)
-    while len(d) > 1 and eq:
-        first = d.pop()
-        last = d.popleft()
-        if first != last:
-            eq = False
-    return eq
-  
 
-def test_palindrome_checker_with_deque():
-    str1 = 'Madam Im Adam'
-    str2 = 'Buffy is a Slayer'
-    assert(palindrome_checker_with_deque(str1) == True)
-    assert(palindrome_checker_with_deque(str2) == False)
-    print('Tests passed!')
+    d1 = Deque()
+    d2 = collections.deque()
+
+    for s in str1.lower():
+        if s not in STRIP:
+            d2.append(s)
+            d1.enqueue(s)
+
+
+    eq1 = True
+    while d1.size() > 1 and eq1:
+        if d1.dequeue_front() != d1.dequeue():
+            eq1 = False
+
+    eq2 = True
+    while len(d2) > 1 and eq2:
+        if d2.pop() != d2.popleft():
+            eq2 = False
+
+    return eq1, eq2
+
+
 
 
 if __name__ == '__main__':
-        test_palindrome_checker_with_deque()   
-
-
+    str1 = 'Madam Im Adam'
+    str2 = 'Buffy is a Slayer'
+    print(palindrome_checker_with_deque(str1))
+    print(palindrome_checker_with_deque(str2))
\ No newline at end of file
diff --git a/src/abstract_structures/queues/queue.py b/src/abstract_structures/queues/queue.py
index 92c9fa9..b3e0df3 100644
--- a/src/abstract_structures/queues/queue.py
+++ b/src/abstract_structures/queues/queue.py
@@ -1,15 +1,16 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
+#!/usr/bin/python
 
+__author__ = "Mari Wahl"
+__email__ = "marina.w4hl@gmail.com"
+
+''' an (inefficient) class for a queue '''
 
 class Queue(object):
-    ''' a class for a queue '''
     def __init__(self):
         self.items = []
 
     def isEmpty(self):
-        return self.items == []
+        return bool(self.items)
 
     def enqueue(self, item):
         self.items.insert(0, item)
@@ -19,21 +20,24 @@ class Queue(object):
 
     def size(self):
         return len(self.items)
-        
+
     def peek(self):
         return self.items[-1]
 
+    def __repr__(self):
+        return '{}'.format(self.items)
 
-def main():
-    queue = Queue()
-    queue.enqueue(1)
-    queue.enqueue(2)
-    queue.enqueue(3)
-    print(queue.size())    
-    print(queue.peek())   
-    print(queue.dequeue())  
-    print(queue.peek())    
 
 
 if __name__ == '__main__':
-    main()
+    queue = Queue()
+    print("Is the queue empty? ", queue.isEmpty())
+    print("Adding 0 to 10 in the queue...")
+    for i in range(10):
+        queue.enqueue(i)
+    print("Queue size: ", queue.size())
+    print("Queue peek : ", queue.peek())
+    print("Dequeue...", queue.dequeue())
+    print("Queue peek: ", queue.peek())
+    print("Is the queue empty? ", queue.isEmpty())
+    print(queue)
\ No newline at end of file
diff --git a/src/abstract_structures/queues/queue_2_stacks.py b/src/abstract_structures/queues/queue_2_stacks.py
deleted file mode 100644
index ee9f185..0000000
--- a/src/abstract_structures/queues/queue_2_stacks.py
+++ /dev/null
@@ -1,60 +0,0 @@
-class MyQueue(object):
-    def __init__(self):
-        self.enq = []
-        self.deq = []
-        
-    def enqueue(self, item):
-        self.enq.append(item)    
-        
-    def _addStacks(self):
-        aux = []
-        if self.enq:
-            while self.enq:
-                aux.append(self.enq.pop())
-        aux.extend(self.deq) 
-        self.deq = aux
-    
-    def dequeue(self):
-        self._addStacks()
-        if self.deq:
-            return self.deq.pop()         
-        raise Exception('Cannot "deque": Queue is empty')
-        
-    def peek(self):
-        if self.deq:
-            return self.deq[-1]
-        elif self.enq:
-            return self.enq[0]
-        raise Exception('Cannot "peek": Queue is empty')
-   
-    def isEmpty(self):
-        return not (bool(self.enq) or bool(self.deq))
-    
-    def size(self):
-        return len(self.enq) + len(self.deq)
-
-    def printQueue(self):
-        self._addStacks()
-        if self.deq:
-            aux = str(self.deq).strip('[]') 
-            return 'Front --> ' + aux +' <-- Back' 
-        else:
-            raise Exception('Cannot "printQueue": Queue is empty')
-          
-
-def main():
-    q = MyQueue()
-    for i in range(1, 11):
-        q.enqueue(i)
-        print(q.printQueue())    
-    assert(q.peek() == 1)
-    assert(q.isEmpty() == False)
-    assert(q.size() == 10)    
-        
-    for i in range (1, 10):
-        q.dequeue()
-        print(q.printQueue()) 
-      
-    
-if __name__ == '__main__':
-    main()
diff --git a/src/abstract_structures/queues/queue_from_two_stacks.py b/src/abstract_structures/queues/queue_from_two_stacks.py
index 6af795d..c36d650 100644
--- a/src/abstract_structures/queues/queue_from_two_stacks.py
+++ b/src/abstract_structures/queues/queue_from_two_stacks.py
@@ -1,48 +1,75 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
+#!/usr/bin/python
+
+__author__ = "Mari Wahl"
+__email__ = "marina.w4hl@gmail.com"
+
+
+''' an example of a queue implemented from 2 stacks '''
 
 
 class Queue(object):
-    ''' an example of a queue implemented from 2 stacks '''
+
     def __init__(self):
         self.in_stack = []
         self.out_stack = []
 
+
+    # basic methods
+    def _transfer(self):
+        while self.in_stack:
+            self.out_stack.append(self.in_stack.pop())
+
+
     def enqueue(self, item):
         return self.in_stack.append(item)
 
+
     def dequeue(self):
+        if not self.out_stack:
+            self._transfer()
         if self.out_stack:
             return self.out_stack.pop()
-        while self.in_stack:
-            self.out_stack.append(self.in_stack.pop())
-        if not self.out_stack: 
+        else:
             return "Queue empty!"
-        return self.out_stack.pop()
-        
+
+
     def size(self):
         return len(self.in_stack) + len(self.out_stack)
 
+
     def peek(self):
+        if not self.out_stack:
+            self._transfer()
         if self.out_stack:
             return self.out_stack[-1]
-        while self.in_stack:
-            self.out_stack.append(self.in_stack.pop())
-        return self.out_stack[-1]      
-   
-        
+        else:
+            return "Queue empty!"
+
+    def __repr__(self):
+        if not self.out_stack:
+            self._transfer()
+        if self.out_stack:
+            return '{}'.format(self.out_stack)
+        else:
+            return "Queue empty!"
+
+    def isEmpty(self):
+        return not (bool(self.in_stack) or bool(self.out_stack))
+
 
-def main():
-    queue = Queue()
-    queue.enqueue(1)
-    queue.enqueue(2)
-    queue.enqueue(3)
-    print(queue.size())    
-    print(queue.peek())   
-    print(queue.dequeue())  
-    print(queue.peek())    
 
 
 if __name__ == '__main__':
-    main()
+    queue = Queue()
+    print("Is the queue empty? ", queue.isEmpty())
+    print("Adding 0 to 10 in the queue...")
+    for i in range(10):
+        queue.enqueue(i)
+    print("Queue size: ", queue.size())
+    print("Queue peek : ", queue.peek())
+    print("Dequeue...", queue.dequeue())
+    print("Queue peek: ", queue.peek())
+    print("Is the queue empty? ", queue.isEmpty())
+
+    print("Printing the queue...")
+    print(queue)
\ No newline at end of file
diff --git a/src/abstract_structures/queues/rotating_array.py b/src/abstract_structures/queues/rotating_array.py
deleted file mode 100644
index 60b3834..0000000
--- a/src/abstract_structures/queues/rotating_array.py
+++ /dev/null
@@ -1,27 +0,0 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
-
-
-def rotating_array(seq, N):
-    ''' rotate an array from right to left for a given number'''
-    myqueue = []
-    for i in range(N):
-        myqueue.append(seq.pop())
-    myqueue.reverse()
-    return myqueue + seq
-
-
-
-def test_rotating_array(module_name='this module'):
-    seq = [1, 2, 3, 4, 5, 6, 7]
-    N = 4
-    assert(rotating_array(seq, N) == [4, 5, 6, 7, 1, 2, 3])
-        
-    s = 'Tests in {name} have {con}!'
-    print(s.format(name=module_name, con='passed'))
-
-
-if __name__ == '__main__':
-    test_rotating_array()
-
diff --git a/src/abstract_structures/stacks/queues_stacks/animal_shelter.py b/src/abstract_structures/stacks/animal_shelter.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/animal_shelter.py
rename to src/abstract_structures/stacks/animal_shelter.py
diff --git a/src/abstract_structures/stacks/queues_stacks/setofStacks.py b/src/abstract_structures/stacks/setofStacks.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/setofStacks.py
rename to src/abstract_structures/stacks/setofStacks.py
diff --git a/src/abstract_structures/stacks/queues_stacks/setofStacksList.py b/src/abstract_structures/stacks/setofStacksList.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/setofStacksList.py
rename to src/abstract_structures/stacks/setofStacksList.py
diff --git a/src/abstract_structures/stacks/queues_stacks/sort_stack.py b/src/abstract_structures/stacks/sort_stack.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/sort_stack.py
rename to src/abstract_structures/stacks/sort_stack.py
diff --git a/src/abstract_structures/stacks/queues_stacks/stackMinElemO1.py b/src/abstract_structures/stacks/stackMinElemO1.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/stackMinElemO1.py
rename to src/abstract_structures/stacks/stackMinElemO1.py
diff --git a/src/abstract_structures/stacks/queues_stacks/stack_.py b/src/abstract_structures/stacks/stack_.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/stack_.py
rename to src/abstract_structures/stacks/stack_.py
diff --git a/src/abstract_structures/stacks/queues_stacks/towers_of_hanoi.py b/src/abstract_structures/stacks/towers_of_hanoi.py
similarity index 100%
rename from src/abstract_structures/stacks/queues_stacks/towers_of_hanoi.py
rename to src/abstract_structures/stacks/towers_of_hanoi.py