License Key Formatting

You are given a license key represented as a string s that consists of only alphanumeric characters and dashes. The string is separated into n + 1 groups by n dashes. You are also given an integer k.

We want to reformat the string s 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 you should convert all lowercase letters to uppercase.

Return the reformatted license key.

Example 1:

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.

Example 2:

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.

Constraints:

• 1 <= s.length <= 105
• s consists of English letters, digits, and dashes '-'.
• 1 <= k <= 104

SOLUTION:

class Solution:
    def licenseKeyFormatting(self, s: str, k: int) -> str:
        op = []
        n = len(s)
        i = 0
        for c in range(n - 1, -1, -1):
            if s[c] != "-":
                i += 1
                op.append(s[c].upper())
                if i % k == 0:
                    op.append("-")
        if len(op) > 0 and op[-1] == "-":
            op.pop()
        op.reverse()
        return "".join(op)