feat(linkedlist): doubly linked list move tail to head

Author: BrianLusina

datastructures/linked_lists/doubly_linked_list/__init__.py

@@ -581,6 +581,28 @@ def move_to_front(self, node: DoubleNode):
             self.head.next = self.tail
             self.tail.previous = self.head
 
+    def move_tail_to_head(self):
+        if self.head and self.head.next:
+            last = self.head
+
+            while last.next:
+                last = last.next
+
+            # we can obtain the second_to_last node from the last node
+            second_to_last = last.previous
+
+            # set the current head node's second_to_last pointer to the last node
+            self.head.previous = last
+            # set the next pointer of the last node to the head node
+            last.next = self.head
+            # remove the second_to_last pointer of the last node
+            last.previous = None
+            # remove the next pointer of the second_to_last node
+            second_to_last.next = None
+
+            # set the head node as the last node
+            self.head = last
+
     def remove_tail(self):
         current = self.head
 

datastructures/linked_lists/doubly_linked_list/test_doubly_linked_list_move_tail_to_head.py

@@ -0,0 +1,57 @@
+import unittest
+
+from . import DoublyLinkedList
+
+
+class DoublyLinkedListMoveTailToHeadTests(unittest.TestCase):
+    def test_1(self):
+        """should move tail of ["r", "a", "c", "e", "c", "a", "r"] to become ["r", "r", "a", "c", "e", "c", "a"]"""
+        linked_list = DoublyLinkedList()
+        data = ["r", "a", "c", "e", "c", "a", "r"]
+        expected = ["r", "r", "a", "c", "e", "c", "a"]
+        expected_head = "r"
+
+        for d in data:
+            linked_list.append(d)
+
+        linked_list.move_tail_to_head()
+        actual_head = linked_list.head
+
+        self.assertIsNotNone(actual_head)
+        self.assertEqual(expected_head, actual_head.data)
+        self.assertIsNone(actual_head.previous)
+
+        actual_data = []
+        while actual_head:
+            actual_data.append(actual_head.data)
+            actual_head = actual_head.next
+
+        self.assertEqual(expected, actual_data)
+
+    def test_2(self):
+        """should move tail of ["a", "b", "c", "d"] to become ["d", "a", "b", "c"]"""
+        linked_list = DoublyLinkedList()
+        data = ["a", "b", "c", "d"]
+        expected = ["d", "a", "b", "c"]
+        expected_head = "d"
+
+        for d in data:
+            linked_list.append(d)
+
+        linked_list.move_tail_to_head()
+        actual_head = linked_list.head
+
+        self.assertIsNotNone(actual_head)
+        self.assertEqual(expected_head, actual_head.data)
+        self.assertIsNone(actual_head.previous)
+
+        actual_data = []
+        while actual_head:
+            actual_data.append(actual_head.data)
+            actual_head = actual_head.next
+
+        self.assertEqual(expected, actual_data)
+
+
+if __name__ == "__main__":
+    unittest.main()

datastructures/linked_lists/singly_linked_list/test_singly_linked_move_tail_to_head.py

@@ -3,7 +3,7 @@
 from . import SinglyLinkedList
 
 
-class LinkedListIsPalindromeTestCase(unittest.TestCase):
+class LinkedListMoveTailToHeadTestCase(unittest.TestCase):
     def test_1(self):
         """should move tail of ["r", "a", "c", "e", "c", "a", "r"] to become ["r", "r", "a", "c", "e", "c", "a"]"""
         linked_list = SinglyLinkedList()