超级大数运算

# 超级大数运算：数组模拟竖式（高精度思想）
def big_add(a, b):
    """两个非负整数（字符串）相加，返回和的字符串"""
    x = [int(c) for c in a[::-1]]   # 倒过来存：个位在 x[0]
    y = [int(c) for c in b[::-1]]
    n = max(len(x), len(y))
    x = x + [0] * (n - len(x))
    y = y + [0] * (n - len(y))
    carry = 0
    res = []
    for i in range(n):
        s = x[i] + y[i] + carry
        res.append(s % 10)
        carry = s // 10
    if carry > 0:
        res.append(carry)
    return "".join(str(d) for d in res[::-1])

def big_mul(a, b):
    """两个非负整数（字符串）相乘，返回积的字符串"""
    x = [int(c) for c in a[::-1]]
    y = [int(c) for c in b[::-1]]
    res = [0] * (len(x) + len(y))
    for i in range(len(x)):
        for j in range(len(y)):
            res[i + j] = res[i + j] + x[i] * y[j]   # 错位相加
    for i in range(len(res) - 1):                   # 统一处理进位
        res[i + 1] = res[i + 1] + res[i] // 10
        res[i] = res[i] % 10
    while len(res) > 1 and res[-1] == 0:            # 去掉前导零
        res.pop()
    return "".join(str(d) for d in res[::-1])

a = "12345678901234567890"
b = "98765432109876543210"
print("a + b =", big_add(a, b))
print("a * b =", big_mul(a, b))

print("——————")

fact = "1"
for i in range(1, 31):
    fact = big_mul(fact, str(i))
print("30! =", fact)