Update 0021-merge-two-sorted-lists.py

python/0021-merge-two-sorted-lists.py

@@ -3,23 +3,32 @@
 #     def __init__(self, val=0, next=None):
 #         self.val = val
 #         self.next = next
+
+# Iterative
 class Solution:
     def mergeTwoLists(self, list1: ListNode, list2: ListNode) -> ListNode:
-        dummy = ListNode()
-        tail = dummy
+        dummy = node = ListNode()
 
         while list1 and list2:
             if list1.val < list2.val:
-                tail.next = list1
+                node.next = list1
                 list1 = list1.next
             else:
-                tail.next = list2
+                node.next = list2
                 list2 = list2.next
-            tail = tail.next
+            node = node.next
 
-        if list1:
-            tail.next = list1
-        elif list2:
-            tail.next = list2
+        node.next = list1 or list2
 
         return dummy.next
+
+# Recursive
+class Solution:
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        if not list1:
+            return list2
+        if not list2:
+            return list1
+        lil, big = (list1, list2) if list1.val < list2.val else (list2, list1)
+        lil.next = self.mergeTwoLists(lil.next, big)
+        return lil