Factorization is the inverse operation of polynomial multiplication. In polynomial multiplication, sorting and simplification often combine several similar items into one item, or two similar items with opposite signs cancel each other out to zero. In the factorization of some polynomials, it is necessary to restore those terms that have been merged or cancelled each other, that is, to split one term in a polynomial into two or more terms, or to add two terms, which only meet the opposite requirements. The former is called a split item and the latter is called an add item.