Tech Base

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].

數字中只要有包含 3、4、7 就代表不能轉 180 度

若是數字中只有 0、1、8 代表該數值轉 180 度後會是同一個數值

所以簡單來說就是要找數值中不包含 3、4、7 且又至少包含一個 2、5、6、9

public int rotatedDigits(int N) {
    int digit, number, count = 0;
    boolean valid;
    for(int i=1; i<=N; i++) {
        valid = false;
        number = i;
        while(number > 0) {
            digit = number % 10;
            if(digit == 3 || digit == 4 || digit == 7) {
                valid = false;
                break;
            } else if (digit == 2 || digit == 5 || digit == 6 || digit == 9) {
                valid = true;
            }

            number /= 10;
        }

        if (valid) {
            count++;
        }
    }

    return count;
}