<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 numTilePossibilities(string tiles) {
        // Count of each alphabet
        std::unordered_map<char, int> count;
        for (const char &c : tiles) {
            count[c]++;
        }
        std::vector<char> chars;
        for (const auto &item : count) {
            chars.push_back(item.first);
        }
        // Find number of sequences of each length
        int ans = 0;
        for (int i{ 1 }; i <= tiles.length(); i++) {
            ans += GetNumSequences(i, chars, count);
        }
        return ans;
    }

    int GetNumSequences(int length, 
        std::vector<char> &chars, std::unordered_map<char, int> &count)
    {
        // Base case
        int ans{ 0 };
        if (length == 1) {
            for (const char &c : chars) {
                if (count[c] > 0) {
                    ans++;
                }
            }
            return ans;
        }
    
        // First letter is 'c' if there is left
        for (const char &c : chars) {
            if (count[c] > 0) {
                count[c]--;
                ans += GetNumSequences(length-1, chars, count);
                count[c]++;
            }
        }
        return ans;
    }
};
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