32. Longest Valid Parentheses

class Solution {
public:
    int longestValidParentheses(string s) {
        //transform the problem into 2 sub problems
        //1. convert string into match pairs
        //2. calculate connectivity from pairs
        
        if (s.length() < 2) {
            return 0;
        } 
        
        //converting matching pair
        vector<int> stack;
        vector<int> pairs;
        for (int i = 0; i < s.length(); i++) {
            if (s[i] == ‘(‘) {
                stack.push_back(i);
            }
            else if (stack.size() > 0) {
                auto top = *(stack.end()-1);
                stack.pop_back();
                pairs.push_back(top);
                pairs.push_back(i);
            }
        }
        
        //check conseqtivity
        std::sort(pairs.begin(), pairs.end());
        if (pairs.size() == 0) {
            return 0;
        }
        int last = pairs[0];
        int cur;
        int acc = 0;
        int m = 0;
        for (int i = 1; i < pairs.size(); i++) {
            cur = pairs[i];
            if (cur - last == 1) {
                acc++;
            } else {
                m = std::max(acc+1, m);
                acc = 0;
            }
            last = cur;
        }
        
        if (acc > 0) {
            m = std::max(acc+1, m);
        }
        
        return m;
    }
};