482. License Key Formatting

class Solution {
    public String licenseKeyFormatting(String S, int K) {
        StringBuilder sb = new StringBuilder(), sb2 = new StringBuilder();
        for(char ch : S.toCharArray())
        {
            if(ch == '-') continue;
            if(Character.isLowerCase(ch)) ch = Character.toUpperCase(ch);
            sb.append(ch);
        }
        int n = sb.length();
        for(int i = n - 1, count = 0; i >= 0; i--)
        {
            sb2.append(sb.charAt(i));
            count++;
            if(count % K == 0)
            {
                sb2.append('-');
                count = 0;
            }
        }
        n = sb2.length();
        if(n > 1 && sb2.charAt(n - 1) == '-') sb2.deleteCharAt(n - 1);
        return sb2.reverse().toString();

    }
}