在 Python 中反转负数

Question

Using slice method in Python we are able to reverse strings and numbers easily. However, how could it be done when the number is negative?

def reverse(x):
    string = str(x)
    return int(string[::-1])

print(reverse(-123))

Gives an error as it returns 321-

EDIT:

Now let's have two more assumptions:

1. If the reversed number is not within [−2^31, 2^31 − 1] it should return 0.

2. For numbers like 120 , it should return 21 as its reverse.

Now, how can we reverse -120 into -21 ?

Answer 1

Assuming that you want to preserve the sign, try this:

def reverse(x):
    ans = int(str(x)[::-1]) if x >= 0 else -int(str(-x)[::-1])
    return ans if -2**31 <= ans <= 2**31 - 1 else 0

It works as expected for all the edge cases introduced by the new requirements:

reverse(321)
=> 123
reverse(-321)
=> -123
reverse(120)
=> 21
reverse(-120)
=> -21
reverse(7463847412)
=> 2147483647
reverse(8463847412)
=> 0
reverse(-8463847412)
=> -2147483648
reverse(-9463847412)
=> 0

Answer 2

I have posted the unwrapped, simplified code to make it easier to understand the answer:

def int_reverser(test):
    if test >= 0:
        answer = int(str(test)[::-1])
    else:
        answer = -int(str(-test)[::-1])
    if -2**31 <= answer <= 2**31 - 1:
        return answer
    else:
        return 0

Answer 3

Edited

You can check ,if your number is negative then add - behind the output. You will get the expected output.
Therefore you code will be like:

def reverse(x):
    y = ""
    if int(x) <= -1:y="-";x = int(x)*-1
    output =  int(y+str(x)[::-1])
    if output >= -2**31 and output <= 2**31 - 1:return output
    else: return 0
    
print(reverse(-123))

Testing:

>>> reverse(-123)
-321
>>> reverse(123)
321
>>> reverse(-120)
-21
>>> reverse(120)
21
>>> reverse(123)
321
>>> reverse(-74634545484744)
0
>>> reverse(74634545484744)
0

It is also very simple and understandable.