<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 res = 0;
        int n = s.size();
        for(int i = 0; i <= n - k; i++){
            unordered_map<char,int> mp;
            int mask = 0;
            int mxidx = i;
            for(int j = i; j < n; ++j){
                mp[s[j]]++;
                if(mp[s[j]] < k){
                    mask |= (1 << (s[j]-'a'));
                }else{
                    mask &= ~(1 << (s[j]-'a'));
                }
                if(mask == 0){
                    if(j-i+1 > res){
                        res = max(res,j-i+1);
                        mxidx = j;
                    }
                    //mxidx = j;
                }
            }
            if(mxidx != i){
                i = mxidx;
            }
        }
        return res;
    }
};
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