public void CommonStart()
        {
            var test1 = new string[] {};
            Assert.AreEqual("", test1.CommonStart());

            var test2 = new[] { "12" };
            Assert.AreEqual("12", test2.CommonStart());

            var test3 = new[] { "", "", "" };
            Assert.AreEqual("", test3.CommonStart());

            var test4 = new[] { "", "12", "" };
            Assert.AreEqual("", test4.CommonStart());

            var test5 = new[] { "12", "12", "123" };
            Assert.AreEqual("12", test5.CommonStart());
        }