Add longest substring k distinct challenge

recursion_dynamic/longest_substr_k_distinct/test_longest_substr.py

@@ -0,0 +1,21 @@
+from nose.tools import assert_equal, assert_raises
+
+
+class TestSolution(object):
+
+    def test_longest_substr(self):
+        solution = Solution()
+        assert_raises(TypeError, solution.longest_substr, None)
+        assert_equal(solution.longest_substr('', k=3), 0)
+        assert_equal(solution.longest_substr('abcabcdefgghiij', k=3), 6)
+        assert_equal(solution.longest_substr('abcabcdefgghighij', k=3), 7)
+        print('Success: test_longest_substr')
+
+
+def main():
+    test = TestSolution()
+    test.test_longest_substr()
+
+
+if __name__ == '__main__':
+    main()