2437. Number of Valid Clock Times

Problem

You are given a string of length 5 called time, representing the current time on a digital clock in the format "hh:mm". The earliest possible time is "00:00" and the latest possible time is "23:59".

In the string time, the digits represented by the ? symbol are unknown, and must be replaced with a digit from 0 to 9.

Return** an integer answer, the number of valid clock times that can be created by replacing every ? with a digit from 0 to **9.

  Example 1:

Input: time = "?5:00"
Output: 2
Explanation: We can replace the ? with either a 0 or 1, producing "05:00" or "15:00". Note that we cannot replace it with a 2, since the time "25:00" is invalid. In total, we have two choices.

Example 2:

Input: time = "0?:0?"
Output: 100
Explanation: Each ? can be replaced by any digit from 0 to 9, so we have 100 total choices.

Example 3:

Input: time = "??:??"
Output: 1440
Explanation: There are 24 possible choices for the hours, and 60 possible choices for the minutes. In total, we have 24 * 60 = 1440 choices.

  Constraints:

• time is a valid string of length 5 in the format "hh:mm".

• "00" <= hh <= "23"

• "00" <= mm <= "59"

• Some of the digits might be replaced with '?' and need to be replaced with digits from 0 to 9.

Solution (Java)

class Solution {
    public int countTime(String time) {
        if (time.equals("??:??")) return 24 * 60;
        int res = 1;

        if (time.charAt(0) == '?' && time.charAt(1) == '?') {
            res *= 24;
        } else if (time.charAt(0) == '?'){
            res *= (time.charAt(1) - '0' >= 4 ? 2 : 3);
        } else if (time.charAt(1) == '?'){
            res *= (time.charAt(0) - '0' >= 2 ? 4 : 10);
        }

        if (time.charAt(4) == '?') res *= 10;
        if (time.charAt(3) == '?') res *= 6;
        return res;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).