<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 {
private:
    void find(vector<string>&arr, unordered_map<char, int>&mp, int start, 
    int dont, int &len, int temp){
        if(start == arr.size()){
            if(temp > len) len = temp;
            return;
        }
        int i;
        unordered_map<char, int>temp2;
        temp2 = mp;
        if(start != dont){
            
            for(i = 0; i < arr[start].size(); i++){
                if(mp[arr[start][i]] == 1){
                    mp = temp2;
                    break;
                }
                mp[arr[start][i]] = 1;
            }
            if(i == arr[start].size()){
                find(arr, mp, start + 1, dont, len, temp + arr[start].size());
            }
        }
        find(arr, temp2, start + 1, dont, len, temp);
        return;
    }
public:
    int maxLength(vector<string>& arr) {
        unordered_map<char, int>mp;
        int len = 0;
        for(int i = 0; i < arr.size(); i++){
            if(i == 0) find(arr, mp, 0, -1, len, 0);
            else find(arr, mp, 0, i, len, 0);
        }
        return len;
    }
};
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