题目

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.

Assume the environment does not allow you to store 64-bit integers (signed or unsigned).

Example 1:

Input: x = 123
Output: 321
Example 2:

Input: x = -123
Output: -321
Example 3:

Input: x = 120
Output: 21
Example 4:

Input: x = 0
Output: 0

Constraints:

-231 <= x <= 231 - 1

解法：模拟

时间复杂度O(n)，空间复杂度O(1)

class Solution {
public:
    int reverse(int x) {
        int r = 0;
        while (x) {
            if (r > 0 && r > (INT_MAX - x % 10) / 10) return 0;
            if (r < 0 && r < (INT_MIN - x % 10) / 10) return 0;
            r = r * 10 + x % 10;
            x /= 10;
        }
        return r;
    }
};