# LeetCode 482. License Key Formatting

## Description

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.

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`

## Explanation

Iterate the characters from the string backward and add a ‘-‘ whenever the count is a multiple of k.

## Python Solution

``````class Solution:
def licenseKeyFormatting(self, s: str, k: int) -> str:
s = s.replace('-', '')

results = []
count = 0

for c in s[::-1]:
if count != 0 and count % k == 0:
results.insert(0, '-')

results.insert(0, c.upper())
count += 1

return "".join(results)``````
• Time Complexity: O(N).
• Space Complexity: O(N).