<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 equal(vector<int> counts1, vector<int> counts2){
        for(int i=0; i<26; i++){
            if(counts1[i]!=counts2[i])
                return false;
        }
        return true;
    } 
    bool checkInclusion(string s1, string s2) {
        vector<int> counts1(26, 0);
        for(int i=0; i<s1.length(); i++)
            counts1[s1[i]-'a']++;
        
        //check for char count in s2
        int i=0;
        vector<int> counts2(26, 0);
        int winsize=s1.length();

        //process first window
        while(i<winsize and i<s2.length()){
            counts2[s2[i]-'a']++;
            i++;
        }

        //check if equal
        if(equal(counts1, counts2))
            return true;

        //process rest
        while(i<s2.length()){
            //add next element to window
            counts2[s2[i]-'a']++;

            //remove leftmost
            char ch=s2[i-winsize];
            counts2[ch-'a']--;
            i++;

            if(equal(counts1, counts2))
            return true;
        }
        return false;
    }
};
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