具体题解见IPad笔记

class Solution {
public:
    int longestValidParentheses(string s) {
        int res = 0;
        stack<int> stk; //这里不要写成char类型，因为进栈的是i是下标，不是s[i]‘（’
        for (int i = 0, start = -1; i < s.size(); i ++) {
            if (s[i] == '(') stk.push(i);
            else {
                if (!stk.empty()) {
                    stk.pop();
                    if (!stk.empty()) res = max(res, i - stk.top());
                    else res = max(res, i - start);
                }
                else start = i;
            }
        }
        return res;
    }
};