10进制与62进制互相转换

Note:这里以`6位编码`为例实现（C#，其他语言类似，或许有更有效的方式），实际中可根据需要调整，不过注意其表述的数值大小范围(eg：6位编码最大值：56800235583(long))。

/// <summary>
/// 将数字转为62进制
/// </summary>
/// <param name="num">待转换的数字</param>
/// <returns>编码字符串</returns>
public static string EncodeStr(long num)
{
int scale = 62;
StringBuilder sb = new StringBuilder();
char[] charArray = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".ToCharArray();

long remainder = 0;

do
{
remainder = num % scale;
sb.Append(charArray[remainder]);
num = num / scale;
}
while (num > scale - 1);

sb.Append(charArray[num]);

char[] chars = sb.ToString().ToCharArray();
Array.Reverse(chars);
string result = new string(chars);

// 6 bits are required, add leading zeros when the encoded string < 6 long
return result.PadLeft(6, '0');
}

/// <summary>
/// 62进制字符串转为数字
/// </summary>
/// <param name="str">编码字符串</param>
/// <returns>解码后的数字</returns>
public static long DecodeNum(string str)
{
int scale = 62;
string charArray = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

// Trim the leading zeros first
str = Regex.Replace(str, "^0*", "");

long num = 0;
int index = 0;
for(int i = 0; i < str.Length; i++)
{
index = charArray.IndexOf(str[i]);
num += (long)(index * (Math.Pow(scale, str.Length - i - 1)));
}

return num;
}