Problem Description
Given a license key string that includes alphanumeric characters and dashes, reformat it so that each group (separated by dashes) contains exactly k characters—except possibly the first group, which may be shorter but must contain at least one character. In addition, all letters must be transformed to uppercase, and extra dashes should be removed.
Key Insights
- Remove all dashes from the string and convert every letter to uppercase.
- Iterate over the cleaned string in reverse to simplify grouping into segments of exactly k characters.
- Insert dashes after every k characters, taking care of the first group which may be shorter.
- Reverse the result to return the correctly ordered reformatted license key.
Space and Time Complexity
Time Complexity: O(n), where n is the length of the input string since we process each character. Space Complexity: O(n), for storing the intermediate cleaned string and the final result.
Solution
We start by cleaning the string: remove all dashes and convert to uppercase. Then, using a reverse iteration allows us to handle the group segmentation without a special case for the first group. We keep a counter to track when to insert a dash after k characters. Once the reverse iteration is complete, the temporarily built string (in reverse order) is reversed again to form the final formatted license key. The algorithm employs a string or list as a mutable collection and uses basic loop constructs, ensuring both time and space remain linear relative to the input size.