482. 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 = 4Output: "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 = 2Output: "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:
The length of string S will not exceed 12,000, and K is a positive integer.
String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
String S is non-empty.

class Solution {    public String licenseKeyFormatting(String S, int K) {        StringBuilder stringBuilder = new StringBuilder();        for (int i = S.length() - 1; i >= 0; i--)            if (S.charAt(i) != '-'){                if( stringBuilder.length() % (K + 1) == K )                    stringBuilder.append('-');                stringBuilder.append(S.charAt(i));            }        return stringBuilder.reverse().toString().toUpperCase();    }}