feat: implement O(1) push_head, pop_tail, and remove handle functions

Author: JanefrancessC

Sprint-2/implement_linked_list/linked_list.py

@@ -0,0 +1,108 @@
+class Node:
+    """
+    A node in a double linked list.
+
+    Each node supports a data, previous, and next
+    - data: the value
+    - previous: pointer to the previous node
+    - next: pointer to the next node
+    """
+
+    def __init__(self, data):
+        self.data = data
+        self.previous = None
+        self.next = None
+
+class LinkedList:
+    """
+    Double Linked List
+
+    Supports O(1) operations on insertion at head, removal from tail, and removal from arbitrary node
+    """
+    def __init__(self):
+        # Head points to the first node in the list
+        self.head = None
+        
+        # Tail points to the last node in the list
+        self.tail = None
+
+    def is_empty(self):
+        """Returns True if the list contains no nodes."""
+        return self.head is None
+
+    def push_head(self, data):
+        """
+        Insert a new node at the head of the list.
+
+        Time Complexity: O(1)
+
+        Returns:
+            Node: reference to the inserted node (handle for later removal)
+        """
+        node = Node(data)
+
+        if self.is_empty():
+            # If list is empty, head and tail both point to new node
+            self.head = node
+            self.tail = node
+        else:
+            # Link new node before current head
+            node.next = self.head
+            self.head.previous = node
+
+            # Update head pointer
+            self.head = node
+
+        return node
+
+    def pop_tail(self):
+        """
+        Remove and return the value at the tail of the list.
+
+        Time Complexity: O(1)
+
+        Raises:
+            ValueError: if the list is empty
+        """
+
+        if self.is_empty():
+            raise ValueError("List is empty.")
+        
+        # store value before removing node
+        value = self.tail.data
+
+        if self.head == self.tail:
+            # Only one element in the list
+            self.head = None
+            self.tail = None
+
+        else:
+            # Move tail pointer backwards
+            self.tail = self.tail.previous
+
+            # Disconnect old tail node
+            self.tail.next = None
+
+        return value
+
+    def remove(self, node):
+        """
+        Remove a node from the list using its reference.
+
+        Time Complexity: O(1)
+
+        Args:
+            node (Node): the node to remove
+        """
+        if node.previous:
+            node.previous.next = node.next
+        else:
+            self.head = node.next
+
+        if node.next:
+            node.next.previous = node.previous
+        else:
+            self.tail = node.previous
+
+        node.previous = None
+        node.next = None