<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:
    def longestSubstring(self, s: str, k: int) -> int:

        @cache
        def rec(start, end): # end is exclusive
            dprint("\ntrying start end", start, end)
            c = Counter(s[start:end])
            badchars = set(char for char, cnt in c.items() if cnt < k)
            if not badchars:
                dprint(f"special return of {end} - {start}")
                return end - start
            totry = set()
            idxstart = start
            idxend = start
            while idxend < end:
                if s[idxend] in badchars:
                    totry.add((idxstart, idxend))
                    idxstart = idxend+1
                    idxend = idxstart
                else:
                    idxend += 1
            totry.add((idxstart, idxend))
            dprint(f"returning max of {totry}")
            out = max(rec(a,b) for a, b in totry)
            dprint("which is ", out)
            return out

        return rec(0, len(s))

def dprint(*args):
    if 0:
        print(*args)
18573