<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 solve(string &tiles, vector<bool> &used, set<string> &ans, string s) {
        if (!s.empty()) {
            ans.insert(s); // Add non-empty string to the set
        }
        
        for (int i = 0; i < tiles.size(); i++) {
            if (!used[i]) { // Ensure the tile hasn't been used yet
                used[i] = true;
                solve(tiles, used, ans, s + tiles[i]); // Recursive call to explore further
                used[i] = false; // Backtrack
            }
        }

        return; // Optional return after loop
    }

    int numTilePossibilities(string tiles) {
        set<string> ans; // Set to store unique combinations
        vector<bool> used(tiles.size(), false); // Track which tiles are used
        solve(tiles, used, ans, ""); // Start the recursive process
        return ans.size(); // Return the count of unique combinations
    }
};

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