Longest Valid Parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

class Solution{
public:
    int longestValidParentheses(string s){
        stack<int> lefts; // index of left parentheses.
        int len = s.length();
        if(len < 2) return 0;

        int start = -1;
        int res = 0;
        for (int i=0;i<len;i++){
            if(s[i] == '('){
                lefts.push(i);
            }else{
                if(lefts.empty()){
                    start = i;
                }else{
                    lefts.pop();
                    if(lefts.empty()){
                        res = max(res,i - start);
                    }else{
                        res = max(res,i- lefts.top());
                    }
                }
            }
        }
        return res;
    }
};