7.5节 练习 第1题：
string sb = "new world day";
string[] str = sb.Split(' ');
string [] str1 = new string[str.Length];
for(int i =0;i<str.Length;i++)
{
str1[i] = str[i].Remove(0, 1);
str[i] = str1[i].Insert(str[i].Length - 1, str[i].Remove(1, str1[i].Length)) + "ay";
}
string sb1 = string.Join(" ", str);
Console.WriteLine(sb1.ToLower());
待续...