update project structure

leetcode_py/21.py

@@ -0,0 +1,25 @@
+# Merge Two Sorted Lists
+
+
+# Definition for singly-linked list
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def mergeTwoLists(self, list1: ListNode, list2: ListNode):
+        prehead = ListNode(-1)
+        prev = prehead
+
+        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