788. Rotated Digits

X is a good number if after rotating each digit individually by 180 degrees, we get a valid number that is different from X. Each digit must be rotated - we cannot choose to leave it alone.

A number is valid if each digit remains a digit after rotation. 0, 1, and 8 rotate to themselves; 2 and 5 rotate to each other; 6 and 9 rotate to each other, and the rest of the numbers do not rotate to any other number and become invalid.

Now given a positive number N, how many numbers X from 1 to N are good?

Example:
Input: 10
Output: 4
Explanation: 
There are four good numbers in the range [1, 10] : 2, 5, 6, 9.
Note that 1 and 10 are not good numbers, since they remain unchanged after rotating.

Note:

N will be in range [1, 10000].

将一个数字的每一位旋转180度，如果跟之前的数字不一样则是有效的

统计1到N中有多少这样的数字

3,4,7旋转后是无效的

C++（5ms）：

 1 class Solution {
 2 public:
 3     int rotatedDigits(int N) {
 4         int res = 0 ;
 5         for(int i = 1 ; i <= N ; i++){
 6             if (isValid(i)){
 7                 res++ ;
 8             }
 9         }
10         return res ;
11     }
12     
13     bool isValid(int n){
14         bool flag = false ;
15         while(n > 0){
16             if (n % 10 == 2) flag = true ;
17             if (n % 10 == 5) flag = true ;
18             if (n % 10 == 6) flag = true ;
19             if (n % 10 == 9) flag = true ;
20             if (n % 10 == 3) return false ;
21             if (n % 10 == 4) return false ;
22             if (n % 10 == 7) return false ;
23             n /= 10 ;
24         }
25         return flag ;
26     }
27 };