<p>You have <code>n</code>&nbsp;&nbsp;<code>tiles</code>, where each tile has one letter <code>tiles[i]</code> printed on it.</p>

<p>Return <em>the number of possible non-empty sequences of letters</em> you can make using the letters printed on those <code>tiles</code>.</p>

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

<pre>
<strong>Input:</strong> tiles = &quot;AAB&quot;
<strong>Output:</strong> 8
<strong>Explanation: </strong>The possible sequences are &quot;A&quot;, &quot;B&quot;, &quot;AA&quot;, &quot;AB&quot;, &quot;BA&quot;, &quot;AAB&quot;, &quot;ABA&quot;, &quot;BAA&quot;.
</pre>

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

<pre>
<strong>Input:</strong> tiles = &quot;AAABBC&quot;
<strong>Output:</strong> 188
</pre>

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

<pre>
<strong>Input:</strong> tiles = &quot;V&quot;
<strong>Output:</strong> 1
</pre>

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

<ul>
	<li><code>1 &lt;= tiles.length &lt;= 7</code></li>
	<li><code>tiles</code> consists of uppercase English letters.</li>
</ul>


class Solution {
public:
  void backtrack(std::set<string> &sequences, string path,
                 std::vector<bool> &used, const std::string_view tiles) {
    if (not path.empty())
      sequences.insert(path);

    for (int i = 0; i < tiles.size(); ++i) {
      if (used[i] or (i > 0 && tiles[i] == tiles[i - 1]) and not used[i - 1]) {
        continue;
      }

      used[i] = true;
      backtrack(sequences, path + tiles[i], used, tiles);
      used[i] = false;
    }
  }

  int numTilePossibilities(string tiles) {    if (tiles.size() == 1)
      return 1;

    std::sort(tiles.begin(), tiles.end());
    std::set<string> sequences{};
    vector<bool> used(tiles.size(), false);
    backtrack(sequences, "", used, tiles);
    return sequences.size();
  }
};

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