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 hold integers within the 32-bit signed integer range. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

（1）思想1：设立一个falg=1，判断x<0，则令flag=-1；然后仍然是将数字转换成字符串，进行str[i] 与 str[len-1-i] 的一个互换，互换完成之后再将字符串转化成数字，此时需要注意的是：数字的范围是-2^32 ~ 2^32-1，所以必须对转换前和转换后的数字均进行判断，在这个范围内，则返回 flag*转换后的数字，否则返回0。

C++：

class Solution {
public:
    int reverse(int x) {
        int flag=1;
        if(x<0)
            flag=-1;
        string str=to_string(abs(x));
        int len=str.size();
        for(int i=0;i<len/2;i++)
        {
            char c=str[i];
            str[i]=str[len-1-i];
            str[len-1-i]=c;
        }
        long int result = flag*atol(str.c_str());
        if(x>pow(2,31)-1 || x<-1*pow(2,31) || result>pow(2,31)-1 || result<-1*pow(2,31))
            return 0;
        return result;
    }
};