fix few details, queues

src/abstract_structures/stacks/animal_shelter.py

@@ -0,0 +1,87 @@
+class Node(object):
+    def __init__(self, name, which):
+        self.name = name
+        self.which = which
+        self.next = next
+        self.timestamp = 0
+
+
+
+class AnimalShelter(object):
+    def __init__(self):
+        self.first_cat = None
+        self.first_dog = None
+        self.last_cat = None
+        self.last_dog = None
+        self.counter = 0
+       
+
+    def enqueue(self, name, which):
+        self.counter += 1        
+        node = Node(name, which)   
+        node.timestamp = self.counter  
+        
+        if which == 'cat':
+            if not self.first_cat:
+                self.first_cat = node
+            if self.last_cat:
+                self.last_cat.next = node
+            self.last_cat = node
+            
+        if which == 'dog':
+            if not self.first_dog:
+                self.first_dog = node
+            if self.last_dog:
+                self.last_dog.next = node
+            self.last_dog = node  
+
+    
+    
+    def dequeueDog(self):
+        if self.first_dog:
+            node = self.first_dog
+            self.first_dog = node.next
+            return str(node.name)
+        raise Exception('No Dogs!')    
+                        
+                      
+                        
+    def dequeueCat(self):
+        if self.first_cat:
+            node = self.first_cat
+            self.first_cat = node.next
+            return str(node.name)
+        raise Exception('No Cats!')       
+
+
+
+    def dequeueAny(self):
+        nodecat = self.first_cat
+        nodedog = self.first_dog
+        if nodecat and not nodedog:
+            return self.dequeueCat()
+        elif nodedog and not nodecat:
+            return self.dequeueDog()
+        elif nodedog and nodecat:
+            if nodedog.timestamp < nodecat.timestamp:
+                return self.dequeueDog()
+            else: 
+                return self.dequeueCat()
+        raise Exception('No Animals!')   
+        
+
+
+
+def main():
+    qs = AnimalShelter()
+    qs.enqueue('bob', 'cat')
+    qs.enqueue('mia', 'cat')
+    qs.enqueue('yoda', 'dog')
+    qs.enqueue('wolf', 'dog')  
+    
+    assert(qs.dequeueDog() == 'yoda')
+    assert(qs.dequeueCat() == 'bob')         
+    print(qs.dequeueAny() == 'mia')     
+
+if __name__ == '__main__':
+    main()