括号序列

import java.util.*;


public class Solution {
    /**
     * 
     * @param s string字符串 
     * @return bool布尔型
     */
    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()|| stack.pop() != s.charAt(i)){
                return false;
            }
        }
        return stack.isEmpty();
    }
}