"Swap Nodes in Pairs" solution

src/swap_nodes_in_pairs.py

@@ -0,0 +1,22 @@
+from utils.linked_list import ListNode
+
+
+class Solution:
+    def swapPairs(self, head: ListNode | None) -> ListNode | None:
+        dummy = ListNode(0)
+        dummy.next = head
+
+        before = dummy
+
+        while before.next and before.next.next:
+            a = before.next
+            b = a.next
+            after = b.next
+
+            a.next = after
+            b.next = a
+            before.next = b
+
+            before = a
+
+        return dummy.next

tests/test_swap_nodes_in_pairs.py

@@ -0,0 +1,18 @@
+import pytest
+
+from src.swap_nodes_in_pairs import Solution
+from src.utils.linked_list import to_linked_list
+from src.utils.linked_list import to_list
+
+
+@pytest.mark.parametrize(
+    "in_list,out_list",
+    (
+        ([1, 2, 3, 4], [2, 1, 4, 3]),
+        ([], []),
+        ([1], [1]),
+    ),
+)
+def test_solution(in_list, out_list):
+    head = to_linked_list(in_list)
+    assert to_list(Solution().swapPairs(head)) == out_list