7. Reverse Integer

描述

Given a 32-bit signed integer, reverse digits of an integer.

Example 1:

Input: 123
Output: 321

Example 2:

Input: -123
Output: -321

Example 3:

Input: 120
Output: 21

解法

转成字符串后翻转再转回来。

• python

class Solution:
    def reverse(self, x: int) -> int:
        sign = 1 if x >= 0 else -1
        res = int(str(abs(x))[::-1])
        return res * sign if res.bit_length() < 32 else 0

使用数学方法，例如 n = 12345，res初始为 0，每次循环为 n 取模加上上次的 res 乘 10，n 每次除以 10 取整。

n = 12345
res = 0

res = 0 * 10 + 12345 % 10 = 5
n = 12345 // 10 = 1234

res = 5 * 10 + 1234 % 10 = 54
n = 1234 // 10 = 123

res = 54 * 10 + 123 % 10 = 543
n = 123 // 10 = 12

res = 543 * 10 + 12 % 10 = 5432
n = 12 // 10 = 1

res = 5432 * 10 + 1 % 10 = 54321
n = 1 // 10 = 0

代码如下：

class Solution:
    def reverse(self, x: int) -> int:
        sign = 1 if x >= 0 else -1
        x = abs(x)
        res = 0

        while x:
            res = res * 10 + x % 10
            x //= 10

        return res * sign if res.bit_length() < 32 else 0

Python 用法总结

整型的 bit_length() 方法：

Help on method_descriptor:

bit_length(...)
    int.bit_length() -> int

    Number of bits necessary to represent self in binary.
    >>> bin(37)
    '0b100101'
    >>> (37).bit_length()
    6