<p>Given a string containing just the characters <code>&#39;(&#39;</code> and <code>&#39;)&#39;</code>, return <em>the length of the longest valid (well-formed) parentheses </em><span data-keyword="substring-nonempty"><em>substring</em></span>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(()&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The longest valid parentheses substring is &quot;()&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;)()())&quot;
<strong>Output:</strong> 4
<strong>Explanation:</strong> The longest valid parentheses substring is &quot;()()&quot;.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;&quot;
<strong>Output:</strong> 0
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= s.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>s[i]</code> is <code>&#39;(&#39;</code>, or <code>&#39;)&#39;</code>.</li>
</ul>

class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int> st;
        int ans = 0;
        for(char c : s) {
            if(c == '(') {
                st.push(-1);
            }
            else {
                int count = 0;
                if(!st.empty()) {
                    if(st.top() == -1) {
                        st.pop();
                        count = 2;
                        while(!st.empty() && st.top() != -1) {
                            count = count + st.top();
                            st.pop();
                        }
                        ans = max(ans, count);
                        st.push(count);
                    }
                    else {
                        count = st.top();
                        st.pop();
                        if(!st.empty()) {
                            st.pop();
                            count = count + 2;
                            while(!st.empty() && st.top() != -1) {
                                count = count + st.top();
                                st.pop();
                            }
                            ans = max(ans, count);
                            st.push(count);
                        }
                    }
                }
            }
        }
        return ans;
    }
};
10400