数据结构实验：实现两表达式算术求和

任务：能够按照指数降序排列建立并输出多项式；

能够完成两个多项式的相加、相减，并将结果输入；

-experimental data structure : achieving two arithmetic expressions Summation tasks : According to index in descending order and the establishment of export polynomial; to complete the combined two polynomials, subtraction, and the results input;