O(n)

class Solution:
    def reverse(self, x: int) -> int:
        if x == 0:
            return 0

        sign = '-' if x < 0 else ''

        x = abs(x)

        l = []
        while x > 0:
            l.append(str(x%10))
            x = int(x/10)

        y = int(sign+''.join(l))

        if y < -2**31 or y > 2**31-1:
            return 0

        return y


def main():
    s = Solution()
    print(s.reverse(123)) # 321
    print(s.reverse(-123)) # -321
    print(s.reverse(120)) # 21
    print(s.reverse(0)) # 0

if __name__ == '__main__':
    main()
			

O(n)

class Solution:
    def reverse(self, x: int) -> int:
        if x == 0:
            return 0

        re = 0
        if x > 0:
            while x > 0:
                re = re*10 + x%10
                x = int(x/10)
        else:
            x = -1*x
            while x > 0:
                re = re*10 + x%10
                x = int(x/10)
            re = -1*re

        if re < -2**31 or re > 2**31-1:
            return 0

        return re


def main():
    s = Solution()
    print(s.reverse(123)) # 321
    print(s.reverse(-123)) # -321
    print(s.reverse(120)) # 21
    print(s.reverse(0)) # 0

if __name__ == '__main__':
    main()
			

O(n)

class Solution:
    def reverse(self, x: int) -> int:
        if x >= 0:
            re = int(str(x)[::-1])
        else:
            re = -1*int(str(-x)[::-1])

        if re < -2**31 or re > 2**31-1:
            return 0

        return re


def main():
    s = Solution()
    print(s.reverse(123)) # 321
    print(s.reverse(-123)) # -321
    print(s.reverse(120)) # 21
    print(s.reverse(0)) # 0

if __name__ == '__main__':
    main()