ReverseInteger

Problem

https://leetcode.com/problems/reverse-integer/

Solution

Extract the sign of x. We can use modulus and division to get the digits of x. Reverse the list of digits. Then check that the digits are in the range \([2^{31} - 1, -2^{31}]\). If so, calculate the reversed integer using the expansion

\[r = \sum_{i=0}^{n-1} 10^i d_i.\]

Code

def reverse(x: int) -> int:
    sign = 1 if x > 0 else -1
    x = sign * x

    digits = []
    while x > 0:
        remainder = x % 10
        digits.append(remainder)
        x = x // 10

    digits = digits[::-1]

    if too_large(sign, digits):
        return 0
    else:
        r = 0
        for i, digit in enumerate(digits):
            r += digit * 10**i
        return sign * r


def too_large(sign, digits):
    if len(digits) < 10:
        return False

    if len(digits) > 10:
        return True

    tl = False
    limit = 2**31 - 1 if sign == 1 else 2**31
    for digit in enumerate(digits):
        limit_digit = limit % 10
        if digit > limit_digit:
            tl = True
        elif digit < limit_digit:
            tl = False

        limit = limit // 10

    return tl

Test

>>> from ReverseInteger import reverse
>>> reverse(123)
321
>>> reverse(-123)
-321
>>> reverse(120)
21

ReverseInteger.reverse(x: int) → int

ReverseInteger.too_large(sign, digits)