Design Phone Directory
<p>Design a phone directory that initially has <code>maxNumbers</code> empty slots that can store numbers. The directory should store numbers, check if a certain slot is empty or not, and empty a given slot.</p>

<p>Implement the <code>PhoneDirectory</code> class:</p>

<ul>
	<li><code>PhoneDirectory(int maxNumbers)</code> Initializes the phone directory with the number of available slots <code>maxNumbers</code>.</li>
	<li><code>int get()</code> Provides a number that is not assigned to anyone. Returns <code>-1</code> if no number is available.</li>
	<li><code>bool check(int number)</code> Returns <code>true</code> if the slot <code>number</code> is available and <code>false</code> otherwise.</li>
	<li><code>void release(int number)</code> Recycles or releases the slot <code>number</code>.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input</strong>
[&quot;PhoneDirectory&quot;, &quot;get&quot;, &quot;get&quot;, &quot;check&quot;, &quot;get&quot;, &quot;check&quot;, &quot;release&quot;, &quot;check&quot;]
[[3], [], [], [2], [], [2], [2], [2]]
<strong>Output</strong>
[null, 0, 1, true, 2, false, null, true]

<strong>Explanation</strong>
PhoneDirectory phoneDirectory = new PhoneDirectory(3);
phoneDirectory.get();      // It can return any available phone number. Here we assume it returns 0.
phoneDirectory.get();      // Assume it returns 1.
phoneDirectory.check(2);   // The number 2 is available, so return true.
phoneDirectory.get();      // It returns 2, the only number that is left.
phoneDirectory.check(2);   // The number 2 is no longer available, so return false.
phoneDirectory.release(2); // Release number 2 back to the pool.
phoneDirectory.check(2);   // Number 2 is available again, return true.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= maxNumbers &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= number &lt; maxNumbers</code></li>
	<li>At most <code>2 * 10<sup>4</sup></code> calls will be made to <code>get</code>, <code>check</code>, and <code>release</code>.</li>
</ul>
Design; Queue; Array; Hash Table; Linked List
class PhoneDirectory {
public:
    PhoneDirectory(int maxNumbers) {
        for (int i = 0; i < maxNumbers; ++i) {
            available.insert(i);
        }
    }

    int get() {
        if (available.empty()) {
            return -1;
        }
        int x = *available.begin();
        available.erase(x);
        return x;
    }

    bool check(int number) {
        return available.contains(number);
    }

    void release(int number) {
        available.insert(number);
    }

private:
    unordered_set<int> available;
};

/**
 * Your PhoneDirectory object will be instantiated and called as such:
 * PhoneDirectory* obj = new PhoneDirectory(maxNumbers);
 * int param_1 = obj->get();
 * bool param_2 = obj->check(number);
 * obj->release(number);
 */