2022.5.28-----leetcode. one thousand and twenty-one

  public String removeOuterParentheses(String ss) {
        String ans="";
        Deque<Character> s=new ArrayDeque<>();
        for(int i=0;i<ss.length();i++){
            char ch=ss.charAt(i);
            if(ch=='('){
                if(!s.isEmpty()) ans+=ch;
                s.add(ch);
            }else{
                s.poll();
               if(!s.isEmpty()) ans+=ch;
            }
        }
        return ans;
    }