LeetCode 482. 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.


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


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).

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