<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) {
        int freq[26];
        for(int i=0; i<26; i++)
            freq[i] = 0;
        for (char ch:s)
            freq[ch-'a']++;
        
        bool s_is_valid = true;;
        int res = 0;
        for (int end=0; end<s.length(); end++) {
            char ch = s[end];
            if (freq[ch-'a'] > 0 && freq[ch-'a'] < k) {
                s_is_valid = false;
                string sub1 = s.substr(0, end);
                string sub2 = s.substr(end+1, s.length()-end);
                return max(longestSubstring(sub1,k), longestSubstring(sub2,k));
            }
        }
        if (s_is_valid) 
            return s.length();
        return 0;

    }
};
88560