<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 {
    unordered_set<string>s;
    vector<bool>vis;
    void helper(string tiles, string curr = "", int ind = 0) {
        if (ind == tiles.length()) {
            s.insert(curr);
            return;
        }
        helper(tiles, curr, ind+1);
        for (int i=0; i<tiles.length(); i++) {
            if (vis[i]) continue;
            curr.push_back(tiles[i]);
            vis[i] = true;
            helper(tiles, curr, ind+1);
            curr.pop_back();
            vis[i] = false;
        }
    }
public:
    int numTilePossibilities(string tiles) {
        int n = tiles.length();
        vis = vector<bool>(n, false);
        helper(tiles);
        return s.size() - 1;
    }
};
18491