7.整数反转

class Solution:
    def reverse(self, x: int) -> int:
        num = 0
        # 取绝对值
        a = abs(x)
        while(a != 0):
            # 首先，假设 a = 123
            # num = 0
            # a = 12
            # num = 3
            # a = 1
            # num = 32
            # a = 0
            # num = 321
            # a=0时，结束循环
            temp = a % 10
            num = num*10 + temp
            a = a//10
        if x > 0 and num <= 2**31 - 1:
            return num
        elif x < 0 and num <= 2**31:
            return -num
        else:
            return 0