20. 有效的括号

    private Map<Character, Character> mappings;
    private Stack<Character> stack;
    {
        mappings = new HashMap<>();
        mappings.put(')', '(');
        mappings.put('}', '{');
        mappings.put(']', '[');
        stack = new Stack<>();
    }

    public boolean isValid(String s) {
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);

            if (mappings.containsKey(c)) {
                char top = stack.empty() ? '#' : stack.pop();
                if (top != mappings.get(c)) {
                    return false;
                }
            } else {
                stack.push(c);
            }
        }
        return stack.empty();
    }