1544. Make The String Great

MakeGood.cs

public class Solution {
  public string MakeGood(string s) {
      if(s.Length <= 1) 
        return s;
      var sb = new StringBuilder(s);
      var i = 1;
      while(i < sb.Length){
        if(Math.Abs(sb[i] - sb[i - 1]) ==  ('a' - 'A')){
          sb.Remove(i, 1);
          sb.Remove(i - 1, 1);
          i = i - 2 < 0 ? 0 : i - 2;
        }
        i++;
      }
      return sb.ToString();
  }
}