题解 | #最长的括号子串#

class Solution {
public:
    /**
     * 
     * @param s string字符串 
     * @return int整型
     */
    int longestValidParentheses(string s) {
        int count = 0, bal = 0;
        int res = 0;
        for (auto &c : s) {
            if (c == '(') {
                ++count;
                ++bal;
            } else {
                --bal;
            }
            if (bal == 0) {
                res = max(res, count * 2);
            }
            if (bal < 0) {
                count = 0;
                bal = 0;
            }
        }
        count = 0, bal = 0;
        for (auto it = s.rbegin(); it != s.rend(); ++it) {
            auto c = *it;
            if (c == ')') {
                ++count;
                ++bal;
            } else {
                --bal;
            }
            if (bal == 0) {
                res = max(res, count * 2);
            }
            if (bal < 0) {
                count = 0;
                bal = 0;
            }
        }
        return res;
    }
};

思路：正反两次遍历+贪心。