Update solution 137 [Python3]

Author: chakyam

solution/137.Single Number II/Solution.py

@@ -0,0 +1,9 @@
+class Solution:
+    def singleNumber(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: int
+        """
+        temp=set(nums)
+        res=(sum(temp)*3-sum(nums))//2
+        return res