public class Solution {
public String licenseKeyFormatting(String S, int K) {
StringBuilder sb = new StringBuilder();
for (int i = S.length() - 1; i >= 0; i--) {
char c = S.charAt(i);
if (c == '-') continue;
sb.append(sb.length() % (K + 1) == K ? "-" : "").append(c);
}
return sb.reverse().toString().toUpperCase();
}
}