<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:

bool isdup(string &s1, string &s2){
    unordered_map<char,int>mp;
    for(char & ch : s1){ // {"aa", "bb"} special case
        if(mp[ch-'a'] > 0){
            return true;
        }
       mp[ch-'a']++;
    }

    for(char & ch: s2){
        if(mp[ch-'a'] > 0){
            return true;
        }
    }

    return false;
}

 int solve(int idx, int n, vector<string>&arr, string temp){
    //base case
     if(idx >= n){
        return temp.length();
     }

     int inc = 0;
     int exc = 0;

     if(isdup(arr[idx], temp)){
        exc = solve(idx+1, n, arr, temp);
     }
     else{
        inc = solve(idx+1, n, arr, temp + arr[idx]);
        exc = solve(idx+1, n,arr, temp);
     }

     return max(inc, exc);
 }
    int maxLength(vector<string>& arr) {
        int n  = arr.size();
         string temp = "";
        return solve(0, n, arr, temp);
    }
};
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