import java.util.*;

public class Solution {
    /**
     *
     * @param s string字符串
     * @return int整型
     */
    public int longestValidParentheses (String s) {
        // write code here
        if (s.length() <= 1) return 0;
        Stack<Integer> stack = new Stack<>();
        int res = 0;
        int start = -1;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                stack.push(i);
            } else {
                if (stack.empty()) {
                    start = i;
                } else {
                    stack.pop();
                    int l = stack.empty() ? start : stack.peek();
                    res = Math.max(res, i - l);
                }
            }
        }
        return res;
    }
}