7. Reverse Integer

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.

Thoughts:

1. Use String:

   1. sign:

   2. reverse abs value: sign * x [::-1]

   3. s * r* (r < 2 ^31)

2. Not use string: Standard way of detecting overflow

class Solution(object):
    def reverse(self, x):
        """
        :type x: int
        :rtype: int
        """
        #sign 
        s = cmp(x,0)
        # reverse abs
        r = int(`s*x`[::-1])
        return s * r * (r < 2 ** 31)

class Solution {
    public int reverse(int x){
        int result = 0;

        while (x != 0)
        {
            int tail = x % 10;
            int newResult = result * 10 + tail;
            if ((newResult - tail) / 10 != result) return 0;
            result = newResult;
            x /= 10;
        }
        return result;
    }
}