7. Reverse Integer

Example 1:
Input: 123
Output: 321

Example 2:
Input: -123
Output: -321

Example 3:
Input: 120
Output: 21
Note:
Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

    public int reverse(int x) {
        long res = 0; //根据note，使用long类型
        while(x != 0) {
            res = res * 10 + x % 10;
            x /= 10;
            if(res < Integer.MIN_VALUE || res > Integer.MAX_VALUE) {
                return 0;
            }
        }
        return (int)res;
    }
}

Time Complexity = O(log(x)) 以10为底
Space Complexity = O(1)