20.有效的括号

输入：s = "()"
输出：true

输入：s = "()[]{}"
输出：true

输入：s = "(]"
输出：false

public static boolean isVaild(String s) {
        // 输入的 s 字符串长度为奇数或第一项是右括号直接 false
        if (s.length() % 2 != 0)
            return false;
        Stack<Character> stack = new Stack<>();
        for (char c : s.toCharArray()) {
            if (c == '(')
                stack.push(')');
            else if (c == '{')
                stack.push('}');
            else if (c == '[')
                stack.push(']');
            else if (stack.empty() || c != stack.pop())
                return false;
        }
        return stack.empty();
    }

else if (stack.empty() || c != stack.pop()) return false;