有约束: 最高位系数不为0

/// Returns the degree of the polynomial.
pub fn degree(&self) -> usize {
    if self.is_zero() {
        0
    } else {
        assert!(self.coeffs.last().map_or(false, |coeff| !coeff.is_zero()));
        self.coeffs.len() - 1
    }
}

但是在做add等计算时，没有很好的处理

fn add(self, other: &'a DensePolynomial<F>) -> DensePolynomial<F> {
    if self.is_zero() {
        other.clone()
    } else if other.is_zero() {
        self.clone()
    } else {
        if self.degree() >= other.degree() {
            let mut result = self.clone();
            for (a, b) in result.coeffs.iter_mut().zip(&other.coeffs) {
                *a += b
            }
            while result.coeffs.last().unwrap().is_zero() {
                result.coeffs.pop();
            }

            result
        } else {
            let mut result = other.clone();
            for (a, b) in result.coeffs.iter_mut().zip(&self.coeffs) {
                *a += b
            }
            // If the leading coefficient ends up being zero, pop it off.
            while result.coeffs.last().unwrap().is_zero() {
                result.coeffs.pop();
            }
            result
        }
    }
}

self.degree() >= other.degree() 没有pop 0系数