<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:
    def maxLength(self, arr: List[str]) -> int:
        length = len(arr)
        self.res = 0
        def helper(currIdx, visited, currLen):
            if currIdx == length:
                self.res = max(self.res, currLen)
                return
            if currIdx != -1:
                currWord = arr[currIdx]
                added = []
                for c in currWord:
                    visited[c] += 1
                    added.append(c)
                    if visited[c] > 1:
                        for c in added:
                            visited[c] -= 1
                            if visited[c] == 0:
                                del visited[c]
                        return
            else:
                currWord = ""
            for i in range(currIdx + 1, length + 1):
                if i == length:
                    helper(i, visited, currLen + len(currWord))
                else:
                    toAdd = True
                    for c in arr[i]:
                        if c in visited:
                            toAdd = False
                            break
                    print(arr[i])
                    print(visited)
                    if toAdd:
                        helper(i, visited, currLen + len(currWord))
            for c in currWord:
                visited[c] -= 1
                if visited[c] == 0:
                    del visited[c]
            return
        helper(-1, defaultdict(int), 0)
        return self.res
203