feat(datastructure): linked list pair wise swap

datastructures/linked_lists/singly_linked_list/__init__.py

@@ -528,7 +528,14 @@ def is_palindrome(self) -> bool:
 
         return True
 
-    def pairwise_swap(self) -> SingleNode:
+    def pairwise_swap(self) -> Optional[SingleNode]:
+        """
+        Swaps nodes in pairs.
+        However, this swaps the values of the nodes in pairs and not the pointers
+
+        Return:
+            SingleNode: new head node or None if no head exists.
+        """
         # nothing to do here
         if not self.head:
             return self.head
@@ -552,6 +559,32 @@ def pairwise_swap(self) -> SingleNode:
         # at this point, the linked list has been swapped in pairs
         return self.head
 
+    def pairwise_swap_two(self) -> Optional[SingleNode]:
+        """
+        Swaps nodes in pairs without swapping the values in the nodes.
+
+        Return:
+            SingleNode: new head node or None if no head exists.
+        """
+        # nothing to do here
+        if not self.head:
+            return self.head
+
+        start = SingleNode(None)
+        start.next = self.head
+        self.head = start
+
+        while self.head.next and self.head.next.next:
+            temp = self.head.next.next
+
+            self.head.next.next = temp.next
+            temp.next = self.head.next
+
+            self.head.next = temp
+            self.head = self.head.next.next
+
+        return start.next
+
     def swap_nodes_at_kth_and_k_plus_1(self, k: int) -> SingleNode:
         a, b = self.head, self.head
 

datastructures/linked_lists/singly_linked_list/test_singly_linked_list_pairwise_swap.py

@@ -0,0 +1,41 @@
+import unittest
+
+from . import SinglyLinkedList, SingleNode
+
+
+class SinglyLinkedListPairwiseTest(unittest.TestCase):
+    def test_empty_list(self):
+        linked_list = SinglyLinkedList()
+        head = linked_list.pairwise_swap()
+        self.assertIsNone(head)
+
+    def test_two(self):
+        """1 -> 2 -> 3 -> 4 should become 2 -> 1 -> 4 -> 3"""
+        linked_list = SinglyLinkedList()
+        linked_list.append(1)
+        linked_list.append(2)
+        linked_list.append(3)
+        linked_list.append(4)
+
+        head = linked_list.pairwise_swap_two()
+
+        expected_head = SingleNode(2, next_=SingleNode(1, next_=SingleNode(4, next_=SingleNode(3))))
+
+        self.assertEqual(head, expected_head)
+
+    def test_three(self):
+        """7 -> 2 -> 1 should become 2 -> 7 -> 1"""
+        linked_list = SinglyLinkedList()
+        linked_list.append(7)
+        linked_list.append(2)
+        linked_list.append(1)
+
+        head = linked_list.pairwise_swap_two()
+
+        expected_head = SingleNode(2, next_=SingleNode(7, next_=SingleNode(1)))
+
+        self.assertEqual(head, expected_head)
+
+
+if __name__ == "__main__":
+    unittest.main()