20. 有效的括号（利用栈的先进后出）

class Solution {
   public static boolean isValid(String s) {
        if (s.length() == 0) {
            return true;
        }
        HashMap<Character, Character> map = new HashMap<>();
        map.put('(', ')');
        map.put('[', ']');
        map.put('{', '}');
        //括号先入后出、遇到左括号压栈，遇到右括号弹出栈顶元素比较
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            if (map.containsKey(s.charAt(i))) {
                stack.push(s.charAt(i));
            } else {
                if (stack.isEmpty() || map.get(stack.pop()) != s.charAt(i)) {
                    return false;
                }
            }
        }
        return stack.isEmpty();
    }
}