如何从字符串创建多项式

class Polynomial:
    def __init__(self, coefficients, exponents):
        self.coefficients = coefficients
        self.exponents = exponents

    def add(self, other):
        # 实现多项式的加法运算
        # ...

    def subtract(self, other):
        # 实现多项式的减法运算
        # ...

    def multiply(self, other):
        # 实现多项式的乘法运算
        # ...

    def to_string(self):
        # 将多项式转换为字符串形式
        # ...

# 示例用法
coefficients = [3, 2, 1]
exponents = [2, 1, 0]
polynomial = Polynomial(coefficients, exponents)
print(polynomial.to_string())  # 输出：3x^2 + 2x + 1