20. 有效的括号

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();

        for (char c : s.toCharArray()) {
            if (c == '(' || c == '[' || c == '{') {
                stack.push(c);
            } else {
                if (stack.isEmpty()) {
                    return true;
                } else if ((c == ')' && stack.pop() != '(') || (c == ']' && stack.pop() != '[') || (c == '}' && stack.pop() != '{')) {
                    return false;
                }
            }
        }

        return stack.isEmpty();
    }
}