<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:
    int maxLength(vector<string>& arr, int currIndex,
                  unordered_map<int, bool> mapping) {
        int n = arr.size();

        if (currIndex == n) {
            return 0;
        }

        string currString = arr[currIndex];

        bool duplicateFound = false;

        for (char x : currString) {
            if (mapping[x]) {
                duplicateFound = true;
                break;
            }
        }

        int withoutInclude = maxLength(arr, currIndex + 1, mapping);
        int withInclude = 0;

        if (!duplicateFound) {
            for (char x : currString) {
                mapping[x] = true;
            }

            withInclude =
                maxLength(arr, currIndex + 1, mapping) + currString.length();
        }

        return max(withInclude, withoutInclude);
    }

    int maxLength(vector<string>& arr) {
        unordered_map<int, bool> mapping;

        vector<string> validStrings;

        for (string x : arr) {
            unordered_map<int, bool> occurence;

            for (char y : x) {
                if (occurence[y]) {
                    break;
                }
            }

            if (occurence.size() == x.size()) {
                validStrings.push_back(x);
            }
        }

        return maxLength(validStrings, 0, mapping);
    }
};
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