<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>

//When you are asked to find the number of possible non-empty sequences of letters, 
//order matters. For example, the sequence "ABC" is different from "BAC," 
//even though both use the same letters. This is a characteristic of permutations.

class Solution {
 void backtrack(string tiles, int level, int &count)
    {
        count++;
        for(int i=level; i<tiles.length(); i++){
            
            if(i!=level && tiles[i]==tiles[level])
                continue;   // to skip same characters
            swap(tiles[i], tiles[level]);
            backtrack(tiles, level+1, count);
        }
    }
    
public:
    int numTilePossibilities(string tiles) {
        
        int count=-1;
        sort(tiles.begin(), tiles.end()); 
        
        backtrack(tiles, 0, count); //level = susbstring length
        
        return count;
    }
};
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