有效括号序列

 public boolean isValid (String s) {
       // write code here
        Stack<Character> stack=new Stack<Character>();
        for (int i=0;i<s.length();i++){
           if(s.charAt(i)=='(') stack.push(')');
           else if(s.charAt(i)=='[') stack.push(']');
           else if(s.charAt(i)=='{') stack.push('}');
           else {
               if(stack.isEmpty()) return false;
               char c=stack.pop();
               if(c!=s.charAt(i)) return false;
           }
        }

        if(stack.isEmpty()) return true;
        else {
            return false;
        }
    }