7. Reverse Integer

题目描述

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

思路

凑出符合要求的string，转成int后return

Detail

1. 判断正负
2. 从后往前loop，从第一个非0的数字开始，append到ans中
3. 如果不为空，且在区间内，返回int(ans)

Complexity

Time: O(n)
Space: O(1)

    def reverse(self, x):
        """
        :type x: int
        :rtype: int
        """
        INT_MAX = 2**31 -1
        INT_MIN = -2**31
        ans = ""
        if x < 0:
            ans += "-"
            x = abs(x)
        string = str(x)[::-1]
        start = False
        for i in string:
            if i != "-" and i != "0":
                start = True
            if start:
                ans += i
        if ans == "" or int(ans) > INT_MAX or int(ans) < INT_MIN:
            return 0
        return int(ans)