Is this an efficient way of solving this problem?

Is this an efficiënt way of solving this problem? :

Look for patterns like "zip" and "zap" in the string -- length-3, starting with 'z' and ending with 'p'. Return a string where for all such words, the middle letter is gone, so "zipXzap" yields "zpXzp".

zipZap("zipXzap") → "zpXzp" zipZap("zopzop") → "zpzp" zipZap("zzzopzop") → "zzzpzp"

I solved it this way, but I'm totally new with stringbuilder. I just googled what was the easiest way to delete characters from a string but then i came to the problem that the indices didn't match anymore and I created a counter to solve this problem. Is this the easiest way to solve this problem?

<p>
public String zipZap(String str) {


  StringBuilder sb = new StringBuilder(str);
  int counter = 0;

  for (int i = 0 ; i<str.length()-2 ; i++) {

    if (str.charAt(i) == 'z' && str.charAt(i+2) == 'p')  {
    sb.deleteCharAt(i+1-counter);
    counter++;
    }


  }

  return sb.toString();

}
</p>