[LeetCode] Reverse Integer

Reverse Integer

Reverse digits of an integer.

Example1: x = 123, return 321
Example2: x = -123, return -321

Have you thought about this?

Here are some good questions to ask before coding. Bonus points for you if you have already thought through this!

If the integer's last digit is 0, what should the output be? ie, cases such as 10, 100.

Did you notice that the reversed integer might overflow? Assume the input is a 32-bit integer, then the reverse of 1000000003 overflows. How should you handle such cases?

For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

Update (2014-11-10):
Test cases had been added to test the overflow behavior.

Solution: The main trick is to deal with overflow. Note that in the following Java code, we may not need String digits. I used it for better code readability. 

public class Solution {
    public int reverse(int x) {
        boolean neg = x < 0? true : false;
        x = Math.abs(x);
        String digits = "";
        while (x > 0) {
            digits = (x % 10) + digits;
            x /= 10;
        }
        int result = 0;
        int mul = 1;
        for (int i = 0; i < digits.length(); i++) {
            int digit = digits.charAt(i) - '0';
            if (
                Integer.MAX_VALUE / mul < digit || 
                Integer.MAX_VALUE - result < mul * digit
            ) {
                return 0;
            }
            result += mul * digit;
            mul *= 10;
        }
        if (neg) result *= -1;
        return result;
    }
}