Latest Time by Replacing Hidden Digits

Problem Description

You are given a string time in the form of hh:mm, where some of the digits in the string are hidden (represented by ?). The valid times are those inclusively between 00:00 and 23:59. Return the latest valid time you can get from time by replacing the hidden digits.

Key Insights

• The hour part can be between 00 and 23.
• The minute part can be between 00 and 59.
• The goal is to maximize the hour and minute by replacing ? with the largest possible digits.
• Careful consideration of the constraints for hours and minutes is required when replacing the ?.

Space and Time Complexity

Time Complexity: O(1) - The length of the string is constant. Space Complexity: O(1) - No additional space is used that grows with input size.

Solution

To solve this problem, we can use a greedy approach. We will iterate over the characters of the time string and replace the '?' with the maximum possible digit based on the constraints of the hour and minute values.

1. For the hour part:

   • If the first character is '?', we can replace it with '2' if the second character is less than '4' or with '1' otherwise.
   • If the second character is '?', we can replace it with '3' if the first character is '2', or with '9' otherwise.

2. For the minute part:

   • The first character can always be replaced with '5' if it is '?'.
   • The second character can be replaced with '9' if it is '?'.

This approach ensures we get the latest valid time possible.

Code Solutions