<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 checkInclusion(std::string s1, std::string s2) {
        std::unordered_multiset<char> pattr;
        //load pattr
        for (auto ch : s1) {
            pattr.insert(ch);
        }

        std::unordered_multiset<char> curr;
        for (uint32_t i = 0, shift = 0; i + shift < s2.size(); ) {
            auto count = pattr.count(s2[i + shift]);
            if (count) {
                curr.insert(s2[i + shift]);
                if (curr.size() < pattr.size()) {
                    shift++;
                }
                else {
                    if (curr == pattr) {
                        return true;
                    }
                    else {
                        curr.erase(curr.find(s2[i]));
                        i++;
                    }
                }
            }
            else {
                curr.clear();
                i = i + shift + 1;
                shift = 0;
            }
        }

        return false;
    }
};
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