在指定的位置插入字符
private string sample(string str, int len, char chr)
{
string result = "" ;
int count = str.Length;
int k = count / len;
if (count <= len) return str;
for (int i = 0; i < k; i++)
{
result += str.Substring(i * len, len) + chr;
}
if (count % len != 0)
{
result += str.Substring(k * len);
}
//result = result.Trim(chr);
return result;
}