7. Reverse Integer

Description

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

Solution

Use long

class Solution {
    public int reverse(int x) {
        int sign = x < 0 ? -1 : 1;
        long a = Math.abs((long) x);
        long b = 0;
        
        while (a > 0) {
            b = 10 * b + a % 10;
            a /= 10;
        }
        
        b *= sign;
        if (b > Integer.MAX_VALUE || b < Integer.MIN_VALUE) {
            return 0;
        }
        
        return (int) b;
    }
}

Detect overflow while iterating

利用overflow会有损耗的特点，通过revert方程来判断。

class Solution {
    public int reverse(int x) {
        int res = 0;
        
        while (x != 0) {
            int tail = x % 10;
            int newRes = 10 * res + tail;
            
            if ((newRes - tail) / 10 != res) {  // overflow
                return 0;
            }
            
            x /= 10;
            res = newRes;
        }
        
        return res;
    }
}