做多项式的减法subtratpolyn(polynomail *pa,polynomail *pb)时可不用重新写函数，可先对多项式的系数进行求反，然后调用加法函数addpolynmail(polynomail *pa,polynomail *pb)；-To do the subtraction polynomial subtratpolyn (polynomail* pa, polynomail* pb) when there is no need to re-write function,