New doubly linkedlist PR: pull/2573 (#3380)

https://github.com/TheAlgorithms/Python/pull/2573 
the second implementation of the Doubly linked list

data_structures/linked_list/doubly_linked_list_two.py

+"""
+- A linked list is similar to an array, it holds values. However, links in a linked
+    list do not have indexes.
+- This is an example of a double ended, doubly linked list.
+- Each link references the next link and the previous one.
+- A Doubly Linked List (DLL) contains an extra pointer, typically called previous
+    pointer, together with next pointer and data which are there in singly linked list.
+ - Advantages over SLL - It can be traversed in both forward and backward direction.
+     Delete operation is more efficient
+"""
+
+
+class Node:
+    def __init__(self, data: int, previous=None, next_node=None):
+        self.data = data
+        self.previous = previous
+        self.next = next_node
+
+    def __str__(self) -> str:
+        return f"{self.data}"
+
+    def get_data(self) -> int:
+        return self.data
+
+    def get_next(self):
+        return self.next
+
+    def get_previous(self):
+        return self.previous
+
+
+class LinkedListIterator:
+    def __init__(self, head):
+        self.current = head
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if not self.current:
+            raise StopIteration
+        else:
+            value = self.current.get_data()
+            self.current = self.current.get_next()
+            return value
+
+
+class LinkedList:
+    def __init__(self):
+        self.head = None  # First node in list
+        self.tail = None  # Last node in list
+
+    def __str__(self):
+        current = self.head
+        nodes = []
+        while current is not None:
+            nodes.append(current.get_data())
+            current = current.get_next()
+        return " ".join(str(node) for node in nodes)
+
+    def __contains__(self, value: int):
+        current = self.head
+        while current:
+            if current.get_data() == value:
+                return True
+            current = current.get_next()
+        return False
+
+    def __iter__(self):
+        return LinkedListIterator(self.head)
+
+    def get_head_data(self):
+        if self.head:
+            return self.head.get_data()
+        return None
+
+    def get_tail_data(self):
+        if self.tail:
+            return self.tail.get_data()
+        return None
+
+    def set_head(self, node: Node) -> None:
+
+        if self.head is None:
+            self.head = node
+            self.tail = node
+        else:
+            self.insert_before_node(self.head, node)
+
+    def set_tail(self, node: Node) -> None:
+        if self.head is None:
+            self.set_head(node)
+        else:
+            self.insert_after_node(self.tail, node)
+
+    def insert(self, value: int) -> None:
+        node = Node(value)
+        if self.head is None:
+            self.set_head(node)
+        else:
+            self.set_tail(node)
+
+    def insert_before_node(self, node: Node, node_to_insert: Node) -> None:
+        node_to_insert.next = node
+        node_to_insert.previous = node.previous
+
+        if node.get_previous() is None:
+            self.head = node_to_insert
+        else:
+            node.previous.next = node_to_insert
+
+        node.previous = node_to_insert
+
+    def insert_after_node(self, node: Node, node_to_insert: Node) -> None:
+        node_to_insert.previous = node
+        node_to_insert.next = node.next
+
+        if node.get_next() is None:
+            self.tail = node_to_insert
+        else:
+            node.next.previous = node_to_insert
+
+        node.next = node_to_insert
+
+    def insert_at_position(self, position: int, value: int) -> None:
+        current_position = 1
+        new_node = Node(value)
+        node = self.head
+        while node:
+            if current_position == position:
+                self.insert_before_node(node, new_node)
+                return None
+            current_position += 1
+            node = node.next
+        self.insert_after_node(self.tail, new_node)
+
+    def get_node(self, item: int) -> Node:
+        node = self.head
+        while node:
+            if node.get_data() == item:
+                return node
+            node = node.get_next()
+        raise Exception("Node not found")
+
+    def delete_value(self, value):
+        node = self.get_node(value)
+
+        if node is not None:
+            if node == self.head:
+                self.head = self.head.get_next()
+
+            if node == self.tail:
+                self.tail = self.tail.get_previous()
+
+            self.remove_node_pointers(node)
+
+    @staticmethod
+    def remove_node_pointers(node: Node) -> None:
+        if node.get_next():
+            node.next.previous = node.previous
+
+        if node.get_previous():
+            node.previous.next = node.next
+
+        node.next = None
+        node.previous = None
+
+    def is_empty(self):
+        return self.head is None
+
+
+def create_linked_list() -> None:
+    """
+    >>> new_linked_list = LinkedList()
+    >>> new_linked_list.get_head_data() is None
+    True
+    >>> new_linked_list.get_tail_data() is None
+    True
+    >>> new_linked_list.is_empty()
+    True
+    >>> new_linked_list.insert(10)
+    >>> new_linked_list.get_head_data()
+    10
+    >>> new_linked_list.get_tail_data()
+    10
+    >>> new_linked_list.insert_at_position(position=3, value=20)
+    >>> new_linked_list.get_head_data()
+    10
+    >>> new_linked_list.get_tail_data()
+    20
+    >>> new_linked_list.set_head(Node(1000))
+    >>> new_linked_list.get_head_data()
+    1000
+    >>> new_linked_list.get_tail_data()
+    20
+    >>> new_linked_list.set_tail(Node(2000))
+    >>> new_linked_list.get_head_data()
+    1000
+    >>> new_linked_list.get_tail_data()
+    2000
+    >>> for value in new_linked_list:
+    ...    print(value)
+    1000
+    10
+    20
+    2000
+    >>> new_linked_list.is_empty()
+    False
+    >>> for value in new_linked_list:
+    ...    print(value)
+    1000
+    10
+    20
+    2000
+    >>> 10 in new_linked_list
+    True
+    >>> new_linked_list.delete_value(value=10)
+    >>> 10 in new_linked_list
+    False
+    >>> new_linked_list.delete_value(value=2000)
+    >>> new_linked_list.get_tail_data()
+    20
+    >>> new_linked_list.delete_value(value=1000)
+    >>> new_linked_list.get_tail_data()
+    20
+    >>> new_linked_list.get_head_data()
+    20
+    >>> for value in new_linked_list:
+    ...    print(value)
+    20
+    >>> new_linked_list.delete_value(value=20)
+    >>> for value in new_linked_list:
+    ...    print(value)
+    >>> for value in range(1,10):
+    ...    new_linked_list.insert(value=value)
+    >>> for value in new_linked_list:
+    ...    print(value)
+    1
+    2
+    3
+    4
+    5
+    6
+    7
+    8
+    9
+    """
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()