Input: ["x"], [] ==> "0"
Input: [], [[3], [-7]] ==> "- 4"
Input: ["x"], [[1,5]] ==> "x^{5}"
Input: ["x"], [[-1,5]] ==> "- x^{5}"
Input: ["x"], [[1,5], [1,5]] ==> "2x^{5}"
Input: ["x"], [[1,5], [-1,5]] ==> "0"
Input: ["x"], [[1,2], [2,1], [3,0]] ==> "3 + 2x + x^{2}"
Input: ["x"], [[3,0], [2,1], [1,2]] ==> "3 + 2x + x^{2}"
Input: ["x", "y", "z"], [[1,0,0,1], [1,1,0,0], [-1,0,1,0]] ==> "x - y + z"
Input: ["x", "y"], [[2,2,2], [3,2,1], [4,2,0], [1,0,0], [-1,1,0],[-2,2,0]] ==>
        "1 - x + 2x^{2} + 3x^{2}y + 2x^{2}y^{2}"