<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) 
    {
        int l=0, r=0, m=0;

        for(int i=0; i<s.length(); i++)
        {
            if(s[i]=='(')
            {
                l++;
            }

            else if(s[i]==')')
            {
                r++;
            }

            if(r==l)
            {
                m = max(m, 2*l);
            }

            else if (r>l)
            {
                l=0;r=0;
            }

            
        } 
        l=0, r=0;
        
        for(int i=s.length()-1; i>=0; i--)
        {
            if(s[i]=='(')
            {
                l++;
            }

            else if(s[i]==')')
            {
                r++;
            }

            if(r==l)
            {
                m = max(m, 2*l);
            }

            else if (r<l)
            {
                l=0;r=0;
            }
        }

        return m;
    }
};
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