Merge pull request doocs#3 from chakyam/master

Update solution 021 [Python3]

Author: yanglbme

solution/021.Merge Two Sorted Lists/Solution.py

@@ -0,0 +1,21 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def mergeTwoLists(self, l1, l2):
+        """
+        :type l1: ListNode
+        :type l2: ListNode
+        :rtype: ListNode
+        """
+        while l1 and l2:
+            if l1.val < l2.val:
+                l1.next=self.mergeTwoLists(l1.next,l2)
+                return l1
+            else:
+                l2.next=self.mergeTwoLists(l1,l2.next)
+                return l2
+        return l1 or l2