adding algo

src/my_project/interviews/top_150_questions_round_20/single_number.py

@@ -0,0 +1,12 @@
+from typing import List, Union, Collection, Mapping, Optional
+from abc import ABC, abstractmethod
+
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
+
+        answer = 0
+
+        for num in nums: 
+            answer ^= num 
+
+        return answer

tests/test_150_questions_round_20/test_single_number_round_20.py

@@ -0,0 +1,11 @@
+import unittest
+from src.my_project.interviews.top_150_questions_round_20\
+.single_number import Solution
+
+class SingleNumberTestCase(unittest.TestCase):
+
+    def test_single_number(self):
+        solution = Solution()
+        output = solution.singleNumber(nums=[2,2,1])
+        target = 1 
+        self.assertEqual(output, target)