7. Reverse Integer（逆反整数序列）

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.

---

反转整数,输入为32位有符号整数，当反转整数溢出的时候返回0。

deque<int> sk;
 int max_int = INT_MAX;

class Solution {
public:
    int reverse(int a) {
        sk.clear();
        if (a > -9 && a < 10)
            return a;
        int  re, last = a;
        bool flag = false;
        if (a < 0){
            last = -a;
            flag = true;
        }
        while (last != 0){
            re = last % 10;
            last = last / 10;
            sk.push_back(re);
        }
        re = 0;
        while (! sk.empty()){
            last = re;
            if (re != 0 &&(max_int / re) < 10)
                return 0;
            re = re * 10 + sk.front();
            sk.pop_front();

        }
        return flag?-re:re;
    }
};

最大的问题是在于计算边界值：我是没有想到好办法，只能使用笨办法

最高效率代码：

class Solution {  
public:  
    int reverse(int x) {  
        // IMPORTANT: Please reset any member data you declared, as  
        // the same Solution instance will be reused for each test case.  
          
        const int max = 0x7fffffff;  //int最大值  
        const int min = 0x80000000;  //int最小值  
        long long sum = 0;   
          
        while(x != 0)  
        {  
            int temp = x % 10;  
            sum = sum * 10 + temp;  
            if (sum > max || sum < min)   //溢出处理  
            {  
                sum = sum > 0 ? max : min;    
                sum=0;  //让sum=0，
                
            }  
            x = x / 10;  
        }  
        return sum;  
    }  
};