Building H2O
<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>

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

<pre>
<strong>Input:</strong> water = &quot;HOH&quot;
<strong>Output:</strong> &quot;HHO&quot;
<strong>Explanation:</strong> &quot;HOH&quot; and &quot;OHH&quot; are also valid answers.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> water = &quot;OOHHHH&quot;
<strong>Output:</strong> &quot;HHOHHO&quot;
<strong>Explanation:</strong> &quot;HOHHHO&quot;, &quot;OHHHHO&quot;, &quot;HHOHOH&quot;, &quot;HOHHOH&quot;, &quot;OHHHOH&quot;, &quot;HHOOHH&quot;, &quot;HOHOHH&quot; and &quot;OHHOHH&quot; are also valid answers.
</pre>

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

<ul>
	<li><code>3 * n == water.length</code></li>
	<li><code>1 &lt;= n &lt;= 20</code></li>
	<li><code>water[i]</code> is either <code>&#39;H&#39;</code> or <code>&#39;O&#39;</code>.</li>
	<li>There will be exactly <code>2 * n</code> <code>&#39;H&#39;</code> in <code>water</code>.</li>
	<li>There will be exactly <code>n</code> <code>&#39;O&#39;</code> in <code>water</code>.</li>
</ul>

[]
[]
[{"value": "cpp", "text": "C++", "defaultCode": "class H2O {\npublic:\n    H2O() {\n        \n    }\n\n    void hydrogen(function<void()> releaseHydrogen) {\n        \n        // releaseHydrogen() outputs \"H\". Do not change or remove this line.\n        releaseHydrogen();\n    }\n\n    void oxygen(function<void()> releaseOxygen) {\n        \n        // releaseOxygen() outputs \"O\". Do not change or remove this line.\n        releaseOxygen();\n    }\n};"}, {"value": "java", "text": "Java", "defaultCode": "class H2O {\n\n    public H2O() {\n        \n    }\n\n    public void hydrogen(Runnable releaseHydrogen) throws InterruptedException {\n\t\t\n        // releaseHydrogen.run() outputs \"H\". Do not change or remove this line.\n        releaseHydrogen.run();\n    }\n\n    public void oxygen(Runnable releaseOxygen) throws InterruptedException {\n        \n        // releaseOxygen.run() outputs \"O\". Do not change or remove this line.\n\t\treleaseOxygen.run();\n    }\n}"}, {"value": "python", "text": "Python", "defaultCode": "class H2O(object):\n    def __init__(self):\n        pass\n\n\n    def hydrogen(self, releaseHydrogen):\n        \"\"\"\n        :type releaseHydrogen: method\n        :rtype: void\n        \"\"\"\n        \n        # releaseHydrogen() outputs \"H\". Do not change or remove this line.\n        releaseHydrogen()\n\n\n    def oxygen(self, releaseOxygen):\n        \"\"\"\n        :type releaseOxygen: method\n        :rtype: void\n        \"\"\"\n        \n        # releaseOxygen() outputs \"O\". Do not change or remove this line.\n        releaseOxygen()"}, {"value": "python3", "text": "Python3", "defaultCode": "class H2O:\n    def __init__(self):\n        pass\n\n\n    def hydrogen(self, releaseHydrogen: 'Callable[[], None]') -> None:\n        \n        # releaseHydrogen() outputs \"H\". Do not change or remove this line.\n        releaseHydrogen()\n\n\n    def oxygen(self, releaseOxygen: 'Callable[[], None]') -> None:\n        \n        # releaseOxygen() outputs \"O\". Do not change or remove this line.\n        releaseOxygen()"}, {"value": "c", "text": "C", "defaultCode": "typedef struct {\n    // User defined data may be declared here.\n    \n} H2O;\n\nH2O* h2oCreate() {\n    H2O* obj = (H2O*) malloc(sizeof(H2O));\n    \n    // Initialize user defined data here.\n    \n    return obj;\n}\n\nvoid hydrogen(H2O* obj) {\n    \n    // releaseHydrogen() outputs \"H\". Do not change or remove this line.\n    releaseHydrogen();\n}\n\nvoid oxygen(H2O* obj) {\n    \n    // releaseOxygen() outputs \"O\". Do not change or remove this line.\n    releaseOxygen();\n}\n\nvoid h2oFree(H2O* obj) {\n    // User defined data may be cleaned up here.\n    \n}"}, {"value": "csharp", "text": "C#", "defaultCode": "public class H2O {\n\n    public H2O() {\n        \n    }\n\n    public void Hydrogen(Action releaseHydrogen) {\n\t\t\n        // releaseHydrogen() outputs \"H\". Do not change or remove this line.\n        releaseHydrogen();\n    }\n\n    public void Oxygen(Action releaseOxygen) {\n        \n        // releaseOxygen() outputs \"O\". Do not change or remove this line.\n\t\treleaseOxygen();\n    }\n}"}]
{
  "name": "H2O",
  "params": [
    {
      "name": "water",
      "type": "string"
    }
  ],
  "return": {
    "type": "string"
  },
  "languages": [
    "java",
    "cpp",
    "python",
    "python3",
    "csharp",
    "c"
  ],
  "manual": true
}
{"cpp": ["C++", "<p>Compiled with <code> clang 19 </code> using the latest C++ 23 standard, and <code>libstdc++</code> provided by GCC 14.</p>\r\n\r\n<p>Your code is compiled with level two optimization (<code>-O2</code>). <a href=\"https://github.com/google/sanitizers/wiki/AddressSanitizer\" target=\"_blank\">AddressSanitizer</a> is also enabled to help detect out-of-bounds and use-after-free bugs.</p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>"], "java": ["Java", "<p><code>OpenJDK 21</code>. Using compile arguments: <code>--enable-preview --release 21</code></p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>\r\n<p>Includes <code>Pair</code> class from https://docs.oracle.com/javase/8/javafx/api/javafx/util/Pair.html.</p>"], "python": ["Python", "<p><code>Python 2.7.18</code>.</p>\r\n\r\n<p>Most libraries are already imported automatically for your convenience, such as <a href=\"https://docs.python.org/2/library/array.html\" target=\"_blank\">array</a>, <a href=\"https://docs.python.org/2/library/bisect.html\" target=\"_blank\">bisect</a>, <a href=\"https://docs.python.org/2/library/collections.html\" target=\"_blank\">collections</a>. If you need more libraries, you can import it yourself.</p>\r\n\r\n<p>For Map/TreeMap data structure, you may use <a href=\"http://www.grantjenks.com/docs/sortedcontainers/\" target=\"_blank\">sortedcontainers</a> library.</p>\r\n\r\n<p>Note that Python 2.7 <a href=\"https://www.python.org/dev/peps/pep-0373/\" target=\"_blank\">is not maintained anymore</a>. For the latest Python, please choose Python3 instead.</p>"], "c": ["C", "<p>Compiled with <code>gcc 14</code> using the gnu11 standard.</p>\r\n\r\n<p>Your code is compiled with level one optimization (<code>-O2</code>). <a href=\"https://github.com/google/sanitizers/wiki/AddressSanitizer\" target=\"_blank\">AddressSanitizer</a> is also enabled to help detect out-of-bounds and use-after-free bugs.</p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>\r\n\r\n<p>For hash table operations, you may use <a href=\"https://troydhanson.github.io/uthash/\" target=\"_blank\">uthash</a>. \"uthash.h\" is included by default. Below are some examples:</p>\r\n\r\n<p><b>1. Adding an item to a hash.</b>\r\n<pre>\r\nstruct hash_entry {\r\n    int id;            /* we'll use this field as the key */\r\n    char name[10];\r\n    UT_hash_handle hh; /* makes this structure hashable */\r\n};\r\n\r\nstruct hash_entry *users = NULL;\r\n\r\nvoid add_user(struct hash_entry *s) {\r\n    HASH_ADD_INT(users, id, s);\r\n}\r\n</pre>\r\n</p>\r\n\r\n<p><b>2. Looking up an item in a hash:</b>\r\n<pre>\r\nstruct hash_entry *find_user(int user_id) {\r\n    struct hash_entry *s;\r\n    HASH_FIND_INT(users, &user_id, s);\r\n    return s;\r\n}\r\n</pre>\r\n</p>\r\n\r\n<p><b>3. Deleting an item in a hash:</b>\r\n<pre>\r\nvoid delete_user(struct hash_entry *user) {\r\n    HASH_DEL(users, user);  \r\n}\r\n</pre>\r\n</p>"], "csharp": ["C#", "<p><a href=\"https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13\" target=\"_blank\">C# 13</a> with .NET 9 runtime</p>"], "python3": ["Python3", "<p><code>Python 3.11</code>.</p>\r\n\r\n<p>Most libraries are already imported automatically for your convenience, such as <a href=\"https://docs.python.org/3/library/array.html\" target=\"_blank\">array</a>, <a href=\"https://docs.python.org/3/library/bisect.html\" target=\"_blank\">bisect</a>, <a href=\"https://docs.python.org/3/library/collections.html\" target=\"_blank\">collections</a>. If you need more libraries, you can import it yourself.</p>\r\n\r\n<p>For Map/TreeMap data structure, you may use <a href=\"http://www.grantjenks.com/docs/sortedcontainers/\" target=\"_blank\">sortedcontainers</a> library.</p>"]}
['Concurrency']