Longest Valid Parentheses.cpp

class Solution {
public:
    int longestValidParentheses(string s) {
        int maxLen = 0, left = 0;
        stack<int> stack;
        for(int i = 0; i < s.size(); ++i)
        {
            if(stack.empty() && s[i] == ')')
            {
                left = i + 1;
                continue;
            }
            if(s[i] == '(')
                stack.push(i);
            else
            {
                stack.pop();
                int len = stack.empty()? i - left + 1: i - stack.top();
                maxLen = max(maxLen, len);
            }
        }
        return maxLen;
    }
};