added bidigrectional linked list

stepik/linked_list/bidirectional_linked_list.py

@@ -0,0 +1,70 @@
+class Node:
+    def __init__(self, value=None, prev_node=None, next_node=None):
+        self.next_node = next_node
+        self.prev_node = prev_node
+        self.value = value
+
+    def __str__(self):
+        return str(self.value)
+
+class List:
+    def __init__(self):
+        self.top = Node()
+
+    def append(self, value):
+        """
+        Добавление нового элемента в конец двунаправленного связного списка.
+        Время работы O(N).
+        """
+
+        current = self.top
+
+        while current.next_node is not None:
+            current = current.next_node
+
+        node = Node(value, current, current.next_node)
+        current.next_node = node
+
+    def append_after_target(self, target, value):
+        """
+        Добавление элемента после некоторого значения target.
+        """
+
+        current = self.top.next_node
+
+        while current is not None:
+            if current.value == target:
+               node = Node(value, current, current.next_node)
+               if current.next_node is not None:
+                   current.next_node.prev_node = node
+               current.next_node = node
+
+            current = current.next_node
+
+    def __str__(self):
+        """
+        Возвращает все элементы связного списка в виде строки.
+        """
+        current = self.top.next_node
+        values = "["
+
+        while current is not None:
+            end = ", " if current.next_node else ""
+            values += str(current) + end
+            current = current.next_node
+
+        return values + "]"
+
+lst = List()
+lst.append(75)
+lst.append(12)
+lst.append(28)
+lst.append(6)
+print(lst)
+
+lst.append_after_target(75, 750)
+lst.append_after_target(12, 120)
+lst.append_after_target(28, 280)
+lst.append_after_target(6, 60)
+
+print(lst)