<p>Given a string <code>s</code> and an integer <code>k</code>, return <em>the length of the longest substring of</em> <code>s</code> <em>such that the frequency of each character in this substring is greater than or equal to</em> <code>k</code>.</p>

<p data-pm-slice="1 1 []">if no such substring exists, return 0.</p>

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

<pre>
<strong>Input:</strong> s = &quot;aaabb&quot;, k = 3
<strong>Output:</strong> 3
<strong>Explanation:</strong> The longest substring is &quot;aaa&quot;, as &#39;a&#39; is repeated 3 times.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;ababbc&quot;, k = 2
<strong>Output:</strong> 5
<strong>Explanation:</strong> The longest substring is &quot;ababb&quot;, as &#39;a&#39; is repeated 2 times and &#39;b&#39; is repeated 3 times.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>4</sup></code></li>
	<li><code>s</code> consists of only lowercase English letters.</li>
	<li><code>1 &lt;= k &lt;= 10<sup>5</sup></code></li>
</ul>

class Solution {
public:
    int longestSubstring(string s, int k) {
        vector<int> count(26,0);
        int n=s.size();
        int res=0;
        int cnt=0;
        unordered_set<char>not_valid;
        for(int i=0;i<n;i++)
        {
            count[int(s[i])-97]++;
        }
        for(int i=0;i<26;i++)
        {
            if(count[i]<k)
                not_valid.insert(char(i+97));
        }
        bool flag=true;
        for(int i=0;i<n;i++)
        {
            if(not_valid.find(s[i])!=not_valid.end())
            {
                res=max(longestSubstring(s.substr(0,i),k),longestSubstring(s.substr(i+1,n-i),k));
                flag=false;
                break;
            }
            
        }  
        if(flag==true)
            return n;      
        return res;
    }
};
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