<p>There are two kinds of threads: <code>oxygen</code> and <code>hydrogen</code>. Your goal is to group these threads to form water molecules.</p>
<p>There is a barrier where each thread has to wait until a complete molecule can be formed. Hydrogen and oxygen threads will be given <code>releaseHydrogen</code> and <code>releaseOxygen</code> methods respectively, which will allow them to pass the barrier. These threads should pass the barrier in groups of three, and they must immediately bond with each other to form a water molecule. You must guarantee that all the threads from one molecule bond before any other threads from the next molecule do.</p>
<p>In other words:</p>
<ul>
<li>If an oxygen thread arrives at the barrier when no hydrogen threads are present, it must wait for two hydrogen threads.</li>
<li>If a hydrogen thread arrives at the barrier when no other threads are present, it must wait for an oxygen thread and another hydrogen thread.</li>
</ul>
<p>We do not have to worry about matching the threads up explicitly; the threads do not necessarily know which other threads they are paired up with. The key is that threads pass the barriers in complete sets; thus, if we examine the sequence of threads that bind and divide them into groups of three, each group should contain one oxygen and two hydrogen threads.</p>
<p>Write synchronization code for oxygen and hydrogen molecules that enforces these constraints.</p>
<strong>Input:</strong> water = "OOHHHH"
<strong>Output:</strong>"HHOHHO"
<strong>Explanation:</strong>"HOHHHO", "OHHHHO", "HHOHOH", "HOHHOH", "OHHHOH", "HHOOHH", "HOHOHH" and "OHHOHH" are also valid answers.
</pre>
<p> </p>
<p><strong>Constraints:</strong></p>
<ul>
<li><code>3 * n == water.length</code></li>
<li><code>1 <= n <= 20</code></li>
<li><code>water[i]</code> is either <code>'H'</code> or <code>'O'</code>.</li>
<li>There will be exactly <code>2 * n</code><code>'H'</code> in <code>water</code>.</li>
<li>There will be exactly <code>n</code><code>'O'</code> in <code>water</code>.</li>
