<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) {
        return longestSubstring(s, k, 0, s.size());
    }

    int longestSubstring(string s, int k, int start, int end) {
     if(start > end) return 0;
        vector<int> cnt(26);
        for(int i = start; i < end; i++) {
            cnt[s[i]-'a']++;
        }
        int left = 0;
        bool isValid = true;
        for(int i = start; i < end; i++) {
            if(cnt[s[i]-'a'] < k && cnt[s[i]-'a'] != 0) {
                left = i;
                isValid = false;
                break;  
            }
        }
        if(isValid) return end - start;
        int l = left > start ? longestSubstring(s, k, start, left) : 0;
        int r = longestSubstring(s, k, left+1, end);
        return max(l, r);
    }
};
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