Reverse Integer

Reverse digits of an integer.

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

Have you thought about this?

Here are some good questions to ask before coding. Bonus points for you if you have already thought through this!

If the integer's last digit is 0, what should the output be? ie, cases such as 10, 100.

Did you notice that the reversed integer might overflow? Assume the input is a 32-bit integer, then the reverse of 1000000003 overflows. How should you handle such cases?

For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

考虑边界条件，开始想先求出最大位，再倒推回去求res。后来看到可以在while里边倒推x，边增加res，更加简洁。int的最大值2的31次方-1，0x7fffffff;最小值为-的2的31次方，0x80000000;

 1 class Solution {
 2 public:
 3     int reverse(int x) {
 4        
 5         const int max=2147483647;
 6         const int min=-2147483648;
 7         long long res=0;
 8         while(x!=0)
 9         {
10             res=res*10+x%10;
11             x=x/10;
12             if(res>max||res<min)
13             {
14                 return 0;
15             }
16         }
17         
18         return res;
19     }
20 };

另一种写法

class Solution {
public:
    int reverse(int x) {
        int res=0;
        while(x!=0)
        {
            if(res>INT_MAX/10) return 0;
            res=res*10+x%10;
            x/=10;
        }
        return res;
    }
};