构造两个按指数递增的有序链表，实现两个一元多项式相加的代码

在构建一个支持指数项的有序链表来表示一元多项式，并实现在链表上进行多项式的加法操作，首先需要定义一个节点结构，包含系数和指数两个字段。然后，我们可以创建两个这样的链表，分别表示两个多项式。当需要将它们相加时，我们需要遍历这两个链表，对应位置的系数相加，同时取较大的指数作为新节点的指数。

以下是使用Python的一种简单实现：

class PolynomialNode:
    def __init__(self, coefficient=0, exponent=0):
        self.coefficient = coefficient
        self.exponent = exponent
        self.next = None

class PolynomialList:
    def __init__(self):
        self.head = None

    def add_term(self, coefficient, exponent):
        if not self.head:
            self.head = PolynomialNode(coefficient, exponent)
        else:
            current = self.head
            while current.next and current.next.exponent >= exponent:
                current = current.next
            new_node = PolynomialNode(coefficient, exponent)
            new_node.next = current.next
            current.next = new_node

# 实现多项式加法
def add_poly_lists(list1, list2):
    result = PolynomialList()
    current1 = list1.head
    current2 = list2.head
    carry = 0

    while current1 or current2:
        coef1 = (current1 and current1.coefficient) or 0
        coef2 = (current2 and current2.coefficient) or 0
        total_coeff = coef1 + coef2 + carry
        carry = total_coeff // (10 ** max(current1.exponent if current1 else 0, current2.exponent if current2 else 0))
        total_coeff %= 10 ** max(current1.exponent if current1 else 0, current2.exponent if current2 else 0)

        result.add_term(total_coeff, max(current1.exponent if current1 else 0, current2.exponent if current2 else 0))

        if current1:
            current1 = current1.next
        if current2:
            current2 = current2.next

    # 如果最后还有进位，则添加一个零次项
    if carry > 0:
        result.add_term(carry, 0)

    return result

# 示例：
list1 = PolynomialList()
list1.add_term(2, 2)  # 2x^2
list1.add_term(4, 1)  # 4x

list2 = PolynomialList()
list2.add_term(5, 1)  # 5x
list2.add_term(6, 0)