string str=string.Empty;
foreach (int n in num)
{
 str+=n.ToString();
}

------解决方案--------------------
C# code

        private String Test(List<int> lisArr)
        {
            if (lisArr == null || lisArr.Count == 0)
            {
                return String.Empty;
            }

            StringBuilder sArr = new StringBuilder();

            foreach (int item in lisArr)
            {
                sArr.Append(item);
            }

            return sArr.ToString();
        }