20. 有效的括号
class Solution {
public boolean isValid(String s) {
Deque<Character> deque=new LinkedList<>();
char ch;
for (int i =0;i<s.length();i++){
ch=s.charAt(i);
if (ch=='('){
deque.push(')');
}
else if (ch=='{'){
deque.push('}');
}
else if (ch=='['){
deque.push(']')