Implement doubly linked list: O(1) operations for head and tail

Author: AFatmaa

Sprint-2/implement_linked_list/linked_list.py

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+class Node:
+    # This class is for the Node (the item).
+    # It keeps the value and pointers to next and previous.
+    def __init__(self, value):
+        self.value = value
+        self.next = None       # Pointer to next node
+        self.previous = None   # Pointer to previous node
+
+
+class LinkedList:
+    # This class controls the list.
+    # It tracks head and tail. operations are O(1).
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+    def push_head(self, value):
+        # Create a new node with the value
+        new_node = Node(value)
+
+        # If list is empty, head and tail is this new node
+        if self.head is None:
+            self.head = new_node
+            self.tail = new_node
+        else:
+            # If list not empty, put new node at start
+            new_node.next = self.head     # Link new node to old head
+            self.head.previous = new_node # Link old head back to new node
+            self.head = new_node          # Update head pointer
+
+        return new_node
+
+    def pop_tail(self):
+        # Remove the last item (tail)
+        if self.tail is None:
+            return None
+
+        value_to_return = self.tail.value
+
+        # Check if only 1 item in list
+        if self.head == self.tail:
+            self.head = None
+            self.tail = None
+        else:
+            # More than 1 item. Move tail pointer back.
+            self.tail = self.tail.previous
+            self.tail.next = None # Cut the link to the old tail
+
+        return value_to_return
+
+    def remove(self, node):
+        # Remove a specific node from the list.
+        # We just assume node is definitely in the list.
+        
+        # If removing the Head node
+        if node == self.head:
+            self.head = node.next
+            if self.head:
+                self.head.previous = None
+            else:
+                self.tail = None # List became empty
+        
+        # If removing the Tail node
+        elif node == self.tail:
+            self.tail = node.previous
+            if self.tail:
+                self.tail.next = None
+            else:
+                self.head = None
+
+        # If removing a node in the middle
+        else:
+            # Link previous node to the next node
+            node.previous.next = node.next
+            # Link next node to the previous node
+            node.next.previous = node.previous