public static int longestValidParentheses1(String s) {
        if (s==null||s.length()==0) {
            return 0;
        }
        int len = s.length();
        int[] dp = new int[len]; // dp[i]代表 i结尾的有效串的长度
        int maxans = 0;
        dp[0] = 0;
        for(int i=1; i<len; i++) {
            if(s.charAt(i) == ')') {
                if(s.charAt(i-1) == '(') {
                    dp[i] = (i>=2?dp[i-2]:0) + 2;
                } else if ((i-dp[i-1]-1>=0) && s.charAt(i-dp[i-1]-1)=='(') {
                    dp[i] = dp[i-1]+2 + (i-dp[i-1]-2>=0?dp[i-dp[i-1]-2]:0);
                }
            }
            maxans = Math.max(maxans, dp[i]);
        }
        return maxans;
    }

/**
     * 栈，保证栈底元素 一定是最后一个未匹配的右括号
     * @param s
     * @return
     */
    public static int longestValidParentheses(String s) {
        int maxans = 0;
        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 {
                    maxans = Math.max(maxans, i-stack.peek());
                }
            }
        }
        return maxans;
    }

public static int longestValidParenthesesX(String s) {
        int left = 0, right =0, maxLenght = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                left++;
            } else {
                right++;
            }
            if (left == right) {
                maxLenght = Math.max(maxLenght, 2*right);
            } else if (right > left) {
                left = right = 0;
            }
        }
        left = right = 0;
        for (int i = s.length()-1; i >=0 ; i--) {
            if (s.charAt(i) == '(') {
                left++;
            } else {
                right++;
            }
            if (left == right) {
                maxLenght = Math.max(maxLenght, 2*left);
            } else if (left > right) {
                left = right = 0;
            }
        }
        return maxLenght;
    }