Main.java:10: error: non-static method cb(List<String>,int,int,int,StringBuilder) cannot be referenced from a static context
    cb(ans,open,close,max,cur);
    ^
Main.java:15: error: cannot find symbol
    	if(cur.length == max*2){
    	      ^
  symbol:   variable length
  location: variable cur of type StringBuilder
Main.java:16: error: incompatible types: unexpected return value
        	return ans.add(cur.toString());
        	              ^
Main.java:21: error: cannot find symbol
        cur.delete(cur.length-1);
                      ^
  symbol:   variable length
  location: variable cur of type StringBuilder
Main.java:26: error: cannot find symbol
        cur.delete(cur.length-1);
                      ^
  symbol:   variable length
  location: variable cur of type StringBuilder
5 errors