大数加法(减法)

1 加法-只支持正整数相加

a = [int(i) for i in list(input())]
b = [int(i) for i in list(input())]

max_len = max(len(a), len(b)) + 1

a = [0] * (max_len - len(a)) + a
b = [0] * (max_len - len(b)) + b

flag = 0
for i in range(len(a) - 1, -1, -1):
    temp = a[i] + b[i] + flag
    if temp > 9:
        temp -= 10
        flag = 1
    else:
        flag = 0
    a[i] = temp
a = [str(i) for i in a]
if a[0] == '0':
    print(''.join(a[1:]))
else:
    print(''.join(a))

2 减法-只支持a>=b

a = [int(i) for i in list(input())]
b = [int(i) for i in list(input())]

max_len = max(len(a), len(b))

a = [0] * (max_len - len(a)) + a
b = [0] * (max_len - len(b)) + b

for i in range(len(a) - 1, -1, -1):
    temp = a[i] - b[i]
    if temp < 0:
        for j in range(i - 1, -1, -1):

            if a[j] - 1 >= 0:
                a[j] = a[j] - 1
                break
            else:
                a[j] = 9
        temp = temp + 10
    a[i] = temp
a = ''.join([str(i) for i in a])
res = a.lstrip('0')

if len(res):
    print(res)
else:
    print('0')

3、 至于任意a+b，a,b可以为负整数可以分开讨论