482. License Key Formatting (Easy)

https://leetcode.com/problems/license-key-formatting/

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 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.

Note:

1. The length of string S will not exceed 12,000, and K is a positive integer.
2. String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
3. String S is non-empty.

Solutions

class Solution {

    // The given string S could only contain dashes which is a corner case.

    public String licenseKeyFormatting(String S, int K) {
        S = S.replace("-", "");
        S = S.toUpperCase();

        int len = S.length();
        if (len <= K) {
            return S;
        }

        String ans = "";
        for (int i = 0; i * K < len; i++) {
            int start = len - (i + 1) * K;
            if (start < 0) {
                start = 0;
            }

            int end = len - i * K;
            ans = S.substring(start, end) + "-" + ans;
        }

        return ans.substring(0, ans.length() - 1);
    }
}

Incorrect Solutions

References