<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) {
        unordered_map<char,int> cnt;
        for (char c : s) cnt[c]++;
        int res = 0;
        bool foundLess = false;
        for (auto [c,v] : cnt) {
            if (v < k) {
                foundLess = true;
                string delim = std::string(1, c);
                vector<string> splits = split(s, delim);
                for (string aSplit : splits) {
                    res = max(res, longestSubstring(aSplit, k));
                }
            }
        }
        if (!foundLess) return s.size();
        return res;
    }

    vector<string> split(string &s, string &delim) {
        int pos = s.find(delim);
        vector<string> res;
        while (pos != std::string::npos) {
            res.push_back(s.substr(0, pos)); 
            s = s.substr(pos + 1); // bug 1
            pos = s.find(delim);
        }
        res.push_back(s); // bug 2: remaining
        return res;
    }
};
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