我用的是两个栈，后面也加了对括号不匹配的判断，但就是死活就是57

static String resolve(String expr) {
        if(expr==null)
            return null;
        Stack<Integer> stack1=new Stack<>();
        Stack<Integer> stack2=new Stack<>();
        for(int i=0;i<expr.length();i++){
            char c=expr.charAt(i);
            if(c!=')'){
                stack1.add((int)c);
            }else{
                if(!stack1.isEmpty()){
                    while(stack1.peek()!=(int)'(')
                        stack2.add(stack1.pop());
                    stack1.pop();
                    while(!stack2.isEmpty()){
                        stack1.add(stack2.pop());
                    }
                }
            }
        }
        String rs="";
        boolean match=true;
        while(!stack1.isEmpty()){
            if(stack1.peek()=='('||stack1.peek()==')'){
                match=false;
                break;
            }
            rs+=stack1.pop();
        }
        return match?rs:"";
    }