将一个十六进制的字符串转换成它的等价的十进制数值。主函数负责收集字符，并判别它们是否为十六进制字符，htoi函数负责转换工作-to a hexadecimal string converted into its equivalent of the decimal value. The main function responsible for the collection of characters, and for judging whether the hexadecimal characters htoi function for conversion work