python3 solution to #21

python3/21.py

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+
+# https://leetcode.com/problems/merge-two-sorted-lists/
+# Iterative solution
+class Solution:
+    def mergeTwoLists(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        tempHead = ListNode() # Empty node to be head of merged list
+        temp = tempHead # End node of merged list, append new nodes to end of temp
+
+        # While BOTH l1 and l2 still have nodes
+        while (l1 is not None and l2 is not None):
+            # If current node of l1 has a smaller value than current node of l2
+            if (l1.val < l2.val):
+                # Append l1 node to merged list
+                temp.next = l1
+                # Go to next l1 node
+                l1 = l1.next
+            else:
+                temp.next = l2
+                l2 = l2.next
+
+            # Go to new end of merged list
+            temp = temp.next
+
+        # Either l1, l2, or neither have nodes left
+        # Can directly append all of the remaining nodes to merged list
+        temp.next = l1 if l1 is not None else l2
+
+        # tempHead is empty node, return the actual head node of merged list
+        return tempHead.next
+