<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:
    bool secondCheck(const string& str, int k) {
        if (str.length() < k) {
            return false;
        }
        vector<int> alphabets(26, 0);
        for (char ch : str) {
            alphabets[ch - 'a']++;
        }
        for (int count : alphabets) {
            if (count > 0 && count < k) {
                return false;
            }
        }
        return true;
    }

    int longestSubstring(string s, int k) {
        int maxLength = 0;
        int n = s.length();

        // Iterate over all possible starting points
        for (int i = 0; i < n; i++) {
            string currentStr = "";
            vector<int> alphabets(26, 0);
            int uniqueChars = 0, charsAtLeastK = 0;

            // Iterate over all possible end points from the starting point i
            for (int j = i; j < n; j++) {
                char ch = s[j];
                currentStr += ch;
                int idx = ch - 'a';

                if (alphabets[idx] == 0) {
                    uniqueChars++;
                }
                alphabets[idx]++;
                if (alphabets[idx] == k) {
                    charsAtLeastK++;
                }

                // Only check if the current substring length is >= k
                if (uniqueChars == charsAtLeastK && j - i + 1 >= k) {
                    maxLength = max(maxLength, j - i + 1);
                }
            }
        }

        return maxLength;
    }
};

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