added 0021_merge_two_sorted_lists.py

Author: nyierr

problems/easy/0021_merge_two_sorted_lists.py

@@ -0,0 +1,48 @@
+"""
+You are given the heads of two sorted linked lists list1 and list2.
+
+Merge the two lists in a one sorted list. The list should be made
+by splicing together the nodes of the first two lists.
+
+Return the head of the merged linked list.
+Example 1:
+  Input: list1 = [1,2,4], list2 = [1,3,4]
+  Output: [1,1,2,3,4,4]
+
+Example 2:
+  Input: list1 = [], list2 = []
+  Output: []
+
+Example 3:
+  Input: list1 = [], list2 = [0]
+  Output: [0]
+
+Constraints:
+  * The number of nodes in both lists is in the range [0, 50]
+  * -100 <= Node.val <= 100
+  * Both list1 and list2 are sorted in non-decreasing order
+"""
+
+class Solution:
+    # O(n+m)
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        if list1 is None and list2 is None:
+            return list1
+        
+        dummy = current = ListNode()
+        while list1 and list2:
+            if list1.val < list2.val:
+                current.next = list1
+                list1 = list1.next
+            else:
+                current.next = list2
+                list2 = list2.next
+            current = current.next
+            
+        if list1 is None:
+            current.next = list2
+        
+        if list2 is None:
+            current.next = list1
+            
+        return dummy.next