public void SingleOrEmpty(int[] array, int expected)
        {
            int result = PermMissingElem_Codility_Easy.ReturnMissingElement(array);

            Assert.Equal(expected, result);
        }
        public void ExampleTests(int[] array, int expected)
        {
            int result = PermMissingElem_Codility_Easy.ReturnMissingElement(array);

            Assert.Equal(expected, result);
        }
        public void MissingFirstOrLast(int[] array, int expected)
        {
            int result = PermMissingElem_Codility_Easy.ReturnMissingElement(array);

            Assert.Equal(expected, result);
        }