<p>Given two strings <code>s1</code> and <code>s2</code>, return <code>true</code><em> if </em><code>s2</code><em> contains a permutation of </em><code>s1</code><em>, or </em><code>false</code><em> otherwise</em>.</p>

<p>In other words, return <code>true</code> if one of <code>s1</code>&#39;s permutations is the substring of <code>s2</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s1 = &quot;ab&quot;, s2 = &quot;eidbaooo&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> s2 contains one permutation of s1 (&quot;ba&quot;).
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s1 = &quot;ab&quot;, s2 = &quot;eidboaoo&quot;
<strong>Output:</strong> false
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s1.length, s2.length &lt;= 10<sup>4</sup></code></li>
	<li><code>s1</code> and <code>s2</code> consist of lowercase English letters.</li>
</ul>

class Solution {
public:
    bool check(vector<int> fre1, vector<int> fre2)
    {
        for(int i = 0; i < 26; i++)
            if(fre1[i] != fre2[i])
                return false;
        return true;
    }
    bool checkInclusion(string s1, string s2) {
        vector<int> freqS1(26, 0);
        vector<int> freqS2(26, 0);

        int n = s1.size(), m = s2.size();
        for(int i = 0; i < n; i++)
            freqS1[s1[i] - 'a']++;
        
        int fast = 0, slow = 0;
        while(fast < m)
        {
            if(!freqS1[s2[fast] - 'a'])
            {
                slow = ++fast;
                for(int i = 0; i < 26; i++)
                    freqS2[i] = 0;
            }
            if(fast >= m)
                break;
            freqS2[s2[fast] - 'a']++;
            
            while(freqS2[s2[fast] - 'a'] > freqS1[s2[fast] - 'a'])
            {
                freqS2[s2[slow] - 'a']--;
                slow ++;
            }

            if(check(freqS1, freqS2))
                return true;

            fast ++;
        }
        

        return false;
    }
};
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