<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 helper(const unordered_map<char,int>& mp, const string& s1)
    {
        unordered_map<char,int> s1_count;
        for(char c : s1) s1_count[c]++;
        for(const auto& pair : s1_count) 
        {
            if(mp.find(pair.first) == mp.end() || mp.at(pair.first) < pair.second)  return false;
        }
        return true;
    }
    bool checkInclusion(string s1, string s2) 
    {
        if(s2.length() < s1.length()) return false;
        unordered_map<char,int>mp;
        int l = 0;
        for(int i = 0; i < s2.length();++i)
        {
            mp[s2[i]]++;
            if(i - l + 1 > s1.length())
            {
                mp[s2[l]]--;
                if(mp[s2[l]] == 0) mp.erase(s2[l]);
                l++;
            }
            if(i - l + 1 == s1.length() && helper(mp, s1)) return true;
        }
        return false;
    }
};
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