2437. Number of Valid Clock Times

T: O(1)

S: O(1)

```java
class Solution {
    public int countTime(String time) {
        int result = 1;
        char first = time.charAt(0);
        char second = time.charAt(1);
        char third = time.charAt(3);
        char fourth = time.charAt(4);

        if (first == '?' && second == '?') {
            result *= 24;
        } else {
            if (first == '?') {
                if ((second - '0') >= 4) {
                    result *= 2;
                } else {
                    result *= 3;
                }
            } else if (second == '?' ) {
                if (first == '0' || first == '1') {
                    result *= 10;
                } else { // first = 2
                    result *= 4;
                }
            }
        }

        if (third == '?') {
            result *= 6;
        }
        if (fourth == '?') {
            result *= 10;
        }    
        return result;
    }
}

/**
??:??


"?5:00"

if second > 4
first can be 0 1
else 012

if second == ?
first can be 0 1 2


"x?:00"
if (first 0 or 1) ? = 0~9
2 => ? = 0~3


third ?
0~5

4th
0~9

23:59

19:59
09:59

 */
```