7. Reverse Integer

Reverse digits of an integer.

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

Note:
The input is assumed to be a 32-bit signed integer. Your function should return 0 when the reversed integer overflows.

反转数字，根据数字的特性 

比如 4321的反转 1234 = （（（1*10 + 2 ）*10）+3）*10+ 4  

class Solution {
public:
    int reverse(int x) {
        long long y = 0;
        int mark = 0;
        if (x < 0) x = -x,mark = 1;
        while(x > 0) {
            y *= 10;
            y += x % 10;
            x /= 10;
            if (y > INT_MAX) return 0; 
        }
        //cout<<((1<<31) - 1)<<" "<<INT_MAX<<endl;
        if (mark) return - y;
        return y;
    }
};