32. Longest Valid Parentheses *HARD*

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.

int longestValidParentheses(string s) {
    int maxLen = 0;
    int lastError = -1;
    vector<int> stack;
    for(int i=0; i<s.size(); i++)
    {
        if (s[i] == '(')
            stack.push_back(i);
        else if (s[i] == ')')
        {
            if (stack.size()>0 )
            {
                stack.pop_back();
                int len;
                if (stack.size()==0)
                    len = i - lastError;
                else
                    len = i - stack.back();
                if (len > maxLen)
                    maxLen = len;
            }
            else
                lastError = i;
        }
    }
    return maxLen;
}