只要求实现加，减，乘三种运算，要求要进行符号运算，求大神帮助，注释最好多一点
http://wenku.baidu.com/link?url=MY99dZu-3T7zFSrtiak7ii1hegbn8C--uwXWuS-XaaRhYjlE2x_zKChUP2llAhn36pXLCcaWvFtSEe_gkIF4qwtfdXs_iuTdninyx-w_2ra
http://download.csdn.net/detail/yorenyoyu/3908648