diff --git a/README.md b/README.md
index f98a9b1..eb3f09d 100644
--- a/README.md
+++ b/README.md
@@ -145,6 +145,7 @@ which is licensed under the GNU Lesser General Public License v3.0 (`LGPL-3.0`).
 |  69  |                         [Sqrt(x)](src/lc0069.c)                          |  Mathematics  |     Square root      | `sqrt`                |
 |  75  |                       [Sort Colors](src/lc0075.c)                        |    Sorting    |         Sort         | `qsort`               |
 | 77 | [Combinations](src/lc0077.c) | Backtracking | Matrix | `CombinationIterator` |
+| 204 | [Count Primes](src/lc0204.c) | Mathematics | Count | `Sieve`, `binary_search_rank` |
 | 273 | [Integer to English Words](src/lc0273.c) | Mathematics | String | `words_to_string` |
 | 367  |                   [Valid Perfect Square](src/lc0367.c)                   |  Mathematics  |       Boolean        | `math_is_polygonal`   |
 | 744 | [Find Smallest Letter Greater Than Target](src/lc0744.c) | Binary search | Character | `binary_search_min` |
diff --git a/lib/binary_search.h b/lib/binary_search.h
index 6786f17..604ede5 100644
--- a/lib/binary_search.h
+++ b/lib/binary_search.h
@@ -9,7 +9,7 @@
  * Searches a sorted collection for the least element greater than or equal to
  * a given value. This function uses the binary search algorithm.
  *
- * @param item     the required minimum value.
+ * @param min      the required minimum value.
  * @param items    the sorted collection of items.
  * @param length   the number of items.
  * @param itemSize the size of each item.
@@ -19,7 +19,26 @@
  *         given minimum.
 */
 Object binary_search_min(
-    Object item,
+    Object min,
+    Array items,
+    size_t length,
+    size_t itemSize,
+    Comparer comparer);
+
+/**
+ * Determines the number of elements less than a given value in a sorted
+ * collection. This function uses the binary search algorithm.
+ * 
+ * @param max      the required maximum value.
+ * @param items    the sorted collection of items.
+ * @param length   the number of items.
+ * @param itemSize the size of each item.
+ * @param comparer the item comparer. The `items` argument must be sorted using
+ *                 the same comparer.
+ * @return The number of items less than the given maximum.
+*/
+size_t binary_search_rank(
+    Object max,
     Array items,
     size_t length,
     size_t itemSize,
diff --git a/lib/combination_iterator.c b/lib/combination_iterator.c
index 872d7e7..8c68e75 100644
--- a/lib/combination_iterator.c
+++ b/lib/combination_iterator.c
@@ -16,7 +16,7 @@ void combination_begin(
     iterator->n = n;
     iterator->end = false;
 
-    for (size_t i = 0; i < iterator->k; i++)
+    for (int i = 0; i < iterator->k; i++)
     {
         iterator->subset[i] = 0;
     }
diff --git a/lib/list.c b/lib/list.c
index a855211..cb524a7 100644
--- a/lib/list.c
+++ b/lib/list.c
@@ -93,11 +93,6 @@ bool list_contains(List instance, Object item, EqualityComparer itemComparer)
     return false;
 }
 
-void list_clear(List instance)
-{
-    instance->count = 0;
-}
-
 void list_reverse(List instance)
 {
     if (!instance->count)
@@ -148,12 +143,17 @@ bool list_sequence_equal(List left, List right, EqualityComparer itemComparer)
     return true;
 }
 
+void list_clear(List instance)
+{
+    instance->count = 0;
+}
+
 void finalize_list(List instance)
 {
+    list_clear(instance);
     free(instance->items);
 
     instance->items = NULL;
     instance->itemSize = 0;
-    instance->count = 0;
     instance->capacity = 0;
 }
diff --git a/lib/list.h b/lib/list.h
index 2b6458d..a6f7faa 100644
--- a/lib/list.h
+++ b/lib/list.h
@@ -63,13 +63,6 @@ Exception list_ensure_capacity(List instance, size_t capacity);
 */
 bool list_contains(List instance, Object item, EqualityComparer itemComparer);
 
-/**
- * Removes all elements from the list.
- * 
- * @param instance the `List` instance.
-*/
-void list_clear(List instance);
-
 /**
  * Reverses the order of the elements in the list.
  * 
@@ -88,6 +81,13 @@ void list_reverse(List instance);
 */
 bool list_sequence_equal(List left, List right, EqualityComparer itemComparer);
 
+/**
+ * Removes all elements from the list.
+ * 
+ * @param instance the `List` instance.
+*/
+void list_clear(List instance);
+
 /**
  * Frees all resources.
  *
diff --git a/lib/priority_queue.c b/lib/priority_queue.c
index e808930..805637d 100644
--- a/lib/priority_queue.c
+++ b/lib/priority_queue.c
@@ -193,8 +193,14 @@ bool priority_queue_try_dequeue(
     return true;
 }
 
+void priority_queue_clear(PriorityQueue instance)
+{
+    instance->count = 0;
+}
+
 void finalize_priority_queue(PriorityQueue instance)
 {
+    priority_queue_clear(instance);
     free(instance->items);
     free(instance->priorities);
 
@@ -202,7 +208,6 @@ void finalize_priority_queue(PriorityQueue instance)
     instance->itemSize = 0;
     instance->priorities = NULL;
     instance->prioritySize = 0;
-    instance->count = 0;
     instance->capacity = 0;
     instance->priorityComparer = NULL;
 }
diff --git a/lib/priority_queue.h b/lib/priority_queue.h
index 419938d..bbef4ca 100644
--- a/lib/priority_queue.h
+++ b/lib/priority_queue.h
@@ -1,5 +1,6 @@
 // Licensed under the MIT License.
 
+#include <stddef.h>
 #include "comparer.h"
 #include "exception.h"
 #include "object.h"
@@ -90,6 +91,13 @@ bool priority_queue_try_dequeue(
     Object item, 
     Object priority);
 
+/**
+ * Removes all elements from the priority queue.
+ * 
+ * @param instance the `PriorityQueue` instance.
+*/
+void priority_queue_clear(PriorityQueue instance);
+
 /**
  * Frees all resources.
  *
diff --git a/src/lc0023.c b/src/lc0023.c
index d0b0d46..3de8616 100644
--- a/src/lc0023.c
+++ b/src/lc0023.c
@@ -75,7 +75,7 @@ struct ListNode* mergeKLists(struct ListNode** lists, int listsSize)
     return head.next;
 }
 
-void finalize()
+void finalizeMergeKLists()
 {
     finalize_priority_queue(&priorityQueue);
 }
diff --git a/src/lc0204.c b/src/lc0204.c
new file mode 100644
index 0000000..a71d7d4
--- /dev/null
+++ b/src/lc0204.c
@@ -0,0 +1,39 @@
+// Licensed under the MIT License.
+
+// Count Primes
+
+#include "../lib/binary_search.h"
+#include "../lib/euler.h"
+#include "../lib/sieve.h"
+
+bool lazy;
+struct Sieve primes;
+
+int countPrimes(int n) 
+{
+    if (n <= 2)
+    {
+        return 0;
+    }
+
+    if (!lazy)
+    {
+        euler_ok(sieve(&primes, 5000000));
+
+        lazy = true;
+    }
+    
+    long long m = n;
+
+    return binary_search_rank(
+        &m, 
+        primes.primes.items, 
+        primes.primes.count, 
+        sizeof(long long),
+        long_long_comparer);
+}
+
+void finalizeCountPrimes()
+{
+    finalize_sieve(&primes);
+}
diff --git a/tools/test.ps1 b/tools/test.ps1
index ca0d048..e2d54a5 100644
--- a/tools/test.ps1
+++ b/tools/test.ps1
@@ -65,14 +65,14 @@ Get-Content '../data/id0081.txt' | ./../id0081.exe
 Get-Content '../data/id0081.txt' | ./../id0082.exe
 Get-Content '../data/id0081.txt' | ./../id0083.exe
 
-for ($i = 84; $i -le 88; $i++)
+for ($i = 84; $i -le 87; $i++)
 {
     Invoke-Expression "./../id00${i}.exe"
 }
 
 ./../id0089.exe
 
-for ($i = 91; $i -le 92; $i++)
+for ($i = 91; $i -le 93; $i++)
 {
     Invoke-Expression "./../id00${i}.exe"
 }
diff --git a/tools/test.sh b/tools/test.sh
index 6d1c92e..6f1aadf 100644
--- a/tools/test.sh
+++ b/tools/test.sh
@@ -72,14 +72,14 @@ cat ../data/id0081.txt | $run ./../id0081
 cat ../data/id0081.txt | $run ./../id0082
 cat ../data/id0081.txt | $run ./../id0083
 
-for i in {0084..0088};
+for i in {0084..0087};
 do
     $run ./../id${i}
 done
 
 $run ./../id0089
 
-for i in {0091..0092};
+for i in {0091..0093};
 do
     $run ./../id${i}
 done