解题思路：

栈

class Solution {
    public int longestValidParentheses(String s) {
        int max = 0;
        // 也可以使用 Stack<Integer> stack=new Stack<>();但Stack是遗留类，不推荐
        Deque<Integer> stack = new LinkedList<>();
        stack.push(-1);
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                stack.push(i);
            } else {
                stack.pop();
                if (stack.isEmpty()) stack.push(i);
                else max = Math.max(max, i - stack.peek());
            }
        }
        return max;
    }
}