题解 | #有效括号序列#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param s string字符串 
     * @return bool布尔型
     */

    private static final Deque<Character> stack = new ArrayDeque<>();

    public boolean isValid (String s) {
        if (s.length() == 0 || s.length() % 2 != 0) return false;
        char first = s.charAt(0);
        if (first == ')' || first == ']' || first == '}') return false;
        stack.push(first);

        for (int i = 1; i != s.length(); ++i) {
            char top = '?';
            if (stack.peek() != null) top = stack.peek();
            if (s.charAt(i) == ')' && top == '(' 
            || s.charAt(i) == ']' && top == '[' 
            || s.charAt(i) == '}' && top == '{') {
                stack.pop();
            } else {
                stack.push(s.charAt(i));
            }
        }
        if (stack.isEmpty()) {
            return true;
        }
        return false;
    }
}