<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:

    int ans=0, n;

    void swap(int i, int j, string& t){
        char temp = t[i];
        t[i]=t[j];
        t[j]=temp;
    }

    void permute(string& t, int i){
        if(i==n-1){
            ans++;
            return;
        }    
        vector<int> freq(26, 0);
        for(int j=i;j<n;j++){
            if(freq[t[j]-'A']==0){
                ans++;
                swap(i,j,t);
                permute(t,i+1);
                swap(i,j,t);
                freq[t[j]-'A']++;
            }
        }
    }

    int numTilePossibilities(string tiles) {
        n=tiles.size();
        permute(tiles,0);
        return ans;
    }
};
12909