<p>You are given an array of strings <code>arr</code>. A string <code>s</code> is formed by the <strong>concatenation</strong> of a <strong>subsequence</strong> of <code>arr</code> that has <strong>unique characters</strong>.</p>

<p>Return <em>the <strong>maximum</strong> possible length</em> of <code>s</code>.</p>

<p>A <strong>subsequence</strong> is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.</p>

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

<pre>
<strong>Input:</strong> arr = [&quot;un&quot;,&quot;iq&quot;,&quot;ue&quot;]
<strong>Output:</strong> 4
<strong>Explanation:</strong> All the valid concatenations are:
- &quot;&quot;
- &quot;un&quot;
- &quot;iq&quot;
- &quot;ue&quot;
- &quot;uniq&quot; (&quot;un&quot; + &quot;iq&quot;)
- &quot;ique&quot; (&quot;iq&quot; + &quot;ue&quot;)
Maximum length is 4.
</pre>

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

<pre>
<strong>Input:</strong> arr = [&quot;cha&quot;,&quot;r&quot;,&quot;act&quot;,&quot;ers&quot;]
<strong>Output:</strong> 6
<strong>Explanation:</strong> Possible longest valid concatenations are &quot;chaers&quot; (&quot;cha&quot; + &quot;ers&quot;) and &quot;acters&quot; (&quot;act&quot; + &quot;ers&quot;).
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> arr = [&quot;abcdefghijklmnopqrstuvwxyz&quot;]
<strong>Output:</strong> 26
<strong>Explanation:</strong> The only string in arr has all 26 characters.
</pre>

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

<ul>
	<li><code>1 &lt;= arr.length &lt;= 16</code></li>
	<li><code>1 &lt;= arr[i].length &lt;= 26</code></li>
	<li><code>arr[i]</code> contains only lowercase English letters.</li>
</ul>

class Solution {
public:
    vector<unordered_set<char>> st;
    int f(int idx,vector<string> &arr,unordered_set<char> &temp){
        int n = arr.size();
        if(idx>=n) return 0;
        int ans = 0;
        for(int j=idx;j<n;j++){
            if(arr[j].size()!=st[j].size()) continue;
            bool flag = true;
            for(auto ch='a';ch<='z';ch++){
                if(temp.find(ch)!=temp.end()&&st[j].find(ch)!=st[j].end()){
                    flag=false;
                    break;
                } 
            }
            if(!flag) continue;
            unordered_set<char> temp1=temp;
            temp.insert(st[j].begin(),st[j].end());
            ans = max(ans,(int)arr[j].size()+f(j+1,arr,temp));
            temp=temp1;
        }
        return ans;
    }
    int maxLength(vector<string>& arr) {
        int n = arr.size();
        st = vector<unordered_set<char>> (n+1);
        for(int i=0;i<n;i++){
            for(auto it:arr[i]){
                st[i].insert(it);
            }
        }
        int ans = 0;
        for(int i=0;i<n;i++){
            if(arr[i].size()!=st[i].size()) continue;
            unordered_set<char> temp(st[i].begin(),st[i].end());
            ans = max(ans,(int)arr[i].size()+f(0,arr,temp));
        }
        return ans;
    }
};
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