(1)建立一元多项式单链表时，根据输入值来确定结束构建单链表； (2)编写一元多项式的相加程序。 (3)要编写多项式单链表的输出函数，以便能验证程序的执行结果。

以下是基于 Python 语言的一元多项式单链表的实现，包括建立、相加和输出函数：

class Node:
    def __init__(self, coef, exp, next=None):
        self.coef = coef # 系数
        self.exp = exp # 指数
        self.next = next # 指向下一个节点的指针

class Polynomial:
    def __init__(self):
        self.head = None # 头节点

    def create(self):
        while True:
            coef = float(input('请输入系数：'))
            exp = int(input('请输入指数：'))
            if coef == 0:
                break
            node = Node(coef, exp)
            if self.head == None:
                self.head = node
            else:
                curr = self.head
                while curr.next != None:
                    curr = curr.next
                curr.next = node

    def add(self, p):
        curr1 = self.head
        curr2 = p.head
        result = Polynomial()
        curr = result.head
        while curr1 != None and curr2 != None:
            if curr1.exp > curr2.exp:
                node = Node(curr1.coef, curr1.exp)
                curr1 = curr1.next
            elif curr1.exp < curr2.exp:
                node = Node(curr2.coef, curr2.exp)
                curr2 = curr2.next
            else:
                coef = curr1.coef + curr2.coef
                if coef != 0:
                    node = Node(coef, curr1.exp)
                curr1 = curr1.next
                curr2 = curr2.next
            if result.head == None:
                result.head = node
                curr = result.head
            else:
                curr.next = node
                curr = curr.next
        while curr1 != None:
            node = Node(curr1.coef, curr1.exp)
            curr1 = curr1.next
            curr.next = node
            curr = curr.next
        while curr2 != None:
            node = Node(curr2.coef, curr2.exp)
            curr2 = curr2.next
            curr.next = node
            curr = curr.next
        return result

    def output(self):
        curr = self.head
        while curr != None:
            print('{:.2f}x^{}'.format(curr.coef, curr.exp), end='')
            curr = curr.next
            if curr != None and curr.coef > 0:
                print('+', end='')
        print()

# 测试代码
p1 = Polynomial()
p1.create()
print('多项式1：', end='')
p1.output()

p2 = Polynomial()
p2.create()
print('多项式2：', end='')
p2.output()

p3 = p1.add(p2)
print('相加后的多项式：', end='')
p3.output()

在测试代码中，首先创建了两个多项式 `p1` 和 `p2`，然后调用相加函数 `add`，将相加后的结果存储在 `p3` 中，并输出三个多项式的结果。其中，输入的系数和指数可以通过 `input()` 函数获取，建议输入浮点数的系数和整数的指数，以便更精确地计算。输出函数中，格式化字符串 `{:.2f}` 表示输出两位小数的浮点数。