1323. Maximum 69 Number

Given a positive integer num consisting only of digits 6 and 9.

Return the maximum number you can get by changing at most one digit (6 becomes 9, and 9 becomes 6).

 

Example 1:

Input: num = 9669
Output: 9969
Explanation: 
Changing the first digit results in 6669.
Changing the second digit results in 9969.
Changing the third digit results in 9699.
Changing the fourth digit results in 9666. 
The maximum number is 9969.
Example 2:

Input: num = 9996
Output: 9999
Explanation: Changing the last digit 6 to 9 results in the maximum number.
Example 3:

Input: num = 9999
Output: 9999
Explanation: It is better not to apply any change.
 

Constraints:

1 <= num <= 10^4
num's digits are 6 or 9.

这题比较简单，从个位数开始判断，看看哪个6最先出现，记录下出现的索引，这样我们就能知道6改为9后的差值了。

class Maximum69Number : public Solution {
public:
    void Exec() {

    }
    int maximum69Number (int num) {
        int last_at = -1, index = 0, res = num;
        while (num != 0) {
            int sub = num % 10;
            if (sub == 6) {
                last_at = index;
            }
            index++;
            num /= 10;
        }
        
        if (last_at != -1) {
            res += 3 * pow(10, last_at);
        }
        return res;
    }
};