升幂：-7+2x^2y +5x^3y-3xy^3 -y^4 或：-7+5x^3y+2x^2y-3xy^3-y^4
降幂：-y^4-3xy^3+5x^3y+2x^2y-7 或：-y^4-3xy^3+2x^2y+5x^3y-7