32. 最长有效括号

输入: "(()"
输出: 2
解释: 最长有效括号子串为 "()"

输入: ")()())"
输出: 4
解释: 最长有效括号子串为 "()()"

public int longestValidParentheses(String s) {
        Stack<Integer> stack = new Stack<Integer>();
        int max = 0;
        int left = -1;
        for (int j = 0; j < s.length(); j++) {
            if (s.charAt(j) == '(') {
                stack.push(j);
            } else {
                if (stack.isEmpty()) {
                    left = j;
                } else {
                    stack.pop();
                    if (stack.isEmpty()) {
                        max = Math.max(max, j - left);
                    } else {
                        max = Math.max(max, j - stack.peek());
                    }
                }
            }
        }
        return max;
    }