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）：

class Solution {
public:
    int rotatedDigits(int N) {
        int res = 0 ;
        for(int i = 1 ; i <= N ; i++){
            if (isValid(i)){
                res++ ;
            }
        }
        return res ;
    }
    
    bool isValid(int n){
        bool flag = false ;
        while(n > 0){
            if (n % 10 == 2) flag = true ;
            if (n % 10 == 5) flag = true ;
            if (n % 10 == 6) flag = true ;
            if (n % 10 == 9) flag = true ;
            if (n % 10 == 3) return false ;
            if (n % 10 == 4) return false ;
            if (n % 10 == 7) return false ;
            n /= 10 ;
        }
        return flag ;
    }
};