diff --git a/leetcode/21.py b/leetcode/21.py
index 4f3417f..b9784fc 100644
--- a/leetcode/21.py
+++ b/leetcode/21.py
@@ -10,14 +10,16 @@ class ListNode:
 
 class Solution:
     def mergeTwoLists(self, list1: ListNode, list2: ListNode):
-        new_list1 = []
-        while list1.next is not None:
-            new_list1.append(list1.val)
-            list1 = list1.next
-        new_list2 = []
-        while list2.next is not None:
-            new_list2.append(list2.val)
-            list2 = list2.next
+        prehead = ListNode(-1)
+        prev = prehead
 
-        new_list = new_list1 + new_list2
-        return new_list.sort()
+        while list1 and list2:
+            if list1.val <= list2.val:
+                prev.next = list1
+                list1 = list1.next
+            else:
+                prev.next = list2
+                list2 = list2.next
+            prev = prev.next
+
+        return prehead.next