[Google Question][LeetCode] License Key Formatting

Problem: You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes.

Given a number K, we would want to reformat the strings such that each group contains exactly K characters, except for the first group which could be shorter than K, but still must contain at least one character. Furthermore, there must be a dash inserted between two groups and all lowercase letters should be converted to uppercase.

Given a non-empty string S and a number K, format the string according to the rules described above.

Example:

Input: S = "5F3Z-2e-9-w", K = 4

Output: "5F3Z-2E9W"

Explanation: The string S has been split into two parts, each part has 4 characters.
Note that the two extra dashes are not needed and can be removed.

Input: S = "2-5g-3-J", K = 2

Output: "2-5G-3J"

Explanation: The string S has been split into three parts, each part has 2 characters except the first part as it could be shorter as mentioned above.

Approach: The approach is straight forward. You can understand it by just looking at the code.

Implementation in C#:

    public string LicenseKeyFormatting(string S, int K) 

    {

        S = S.Replace("-", "");

        if (string.IsNullOrWhiteSpace(S))

        {

            return "";

        }

        int length = S.Length;

        int charsinFirstGroup = length % K == 0 ? K : length % K;

        StringBuilder builder = new StringBuilder();

        int j = 0;

        for (int i = 0; i < charsinFirstGroup; ++i)

        {

            builder.Append(char.ToUpper(S[j++]));

        }

        builder.Append('-');

        while (j < length)

        {

            for (int i = 0; i < K; ++i)

            {

                builder.Append(char.ToUpper(S[j++]));

            }

            builder.Append('-');

        }

        builder.Length--;

        return builder.ToString();

    }

Complexity: O(n)