有效的括号

class Solution {
    Stack<Character> stk = new Stack<>();
    public boolean isValid(String s) {
        int len=s.length();
        for(int i=0;i<len;i++){
            char ch = s.charAt(i);
            if(stk.size()==0){
                if(ch=='}'||ch==')'||ch==']') return false;//"}}}"
            }
            if(ch=='{'||ch=='['||ch=='('){
                stk.push(ch);
            }else{
                if(ch==')'&&stk.peek()=='(') stk.pop();
                else if(ch=='}'&&stk.peek()=='{') stk.pop();
                else if(ch==']'&&stk.peek()=='[') stk.pop();
                else return false;
            }
        }
        return stk.size()==0?true:false;

    }
}