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
