<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(string s1, string s2) {
        
        int n1 = s1.length();
        
        int n2 = s2.length();
        
        unordered_set<char> def( s1.begin(), s1.end());
        
        unordered_map<char,int> mp;
        
        for( char ch: s1) {
            mp[ch] += 1;
        }
        
        int i = 0;
        int j = -1;
        
        auto add = [&]( char ch)->void {
            mp[ch] -= 1;
            if(mp[ch]==0) {
                def.erase(ch);
            }
        };
        
        auto rem = [&]( char ch)->void {
            mp[ch] += 1;
            if(mp[ch]==1) {
                def.insert(ch);
            }
        };
        
        
        while(i<n2) {
            if(i>j) {
                j = i;
                if(mp.find(s2[i])==mp.end()) {
                    i++;
                    continue;
                } else {
                    add(s2[i]);
                }
            }
            // cout << " i : " << i << " j : " << j << endl;
            // cout << " def: ";
            // for( char ch: def) {
            //     cout << ch;
            // }
            // cout << endl;
            // for( auto p: mp) {
            //     cout << "(" << p.first << "," << p.second << ")";
            // }
            // cout << endl;
            if(def.empty()) {
                return true;
            } else {
                if(j+1>=n2) {
                    break;
                }
                if((mp.find(s2[j+1])!=mp.end()) && (mp[s2[j+1]]>0)) {
                    j++;
                    add(s2[j]);
                } else {
                    rem(s2[i]);
                    i++;
                }
            }
        }
        
        if(def.empty()) {
            return true;
        }
        
        
        return false;
    }
};
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