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;}
}