class Solution {public:    int longestValidParentheses(string s) {        vector<int> stack;        int maxlen = 0;        int curlen = 0;        int last = -1;        int len = s.length();        for (int i=0; i<len; i++) {            char ch = s[i];            if (ch == ‘(‘) {                stack.push_back(i);                continue;            }            if (stack.empty()) {                last = i;                continue;            }            stack.pop_back();            if (stack.empty()) {                curlen = i - last;            } else {                curlen = i - stack.back();            }                        if (curlen > maxlen) maxlen = curlen;        }                return maxlen;    }};