diff --git a/challenges/heap-algorithm/README.md b/challenges/heap-algorithm/README.md
index be40826..d42c990 100644
--- a/challenges/heap-algorithm/README.md
+++ b/challenges/heap-algorithm/README.md
@@ -6,6 +6,8 @@ Created by [@theoludwig](https://github.com/theoludwig) on 8 November 2021.
 
 Write a program that generates all possible unique permutations of a string.
 
+The order of the generated permutations is important, see the example below.
+
 ## Source
 
 - [Heap's Algorithm - Wikipedia](https://en.wikipedia.org/wiki/Heap%27s_algorithm)
@@ -25,10 +27,10 @@ abc
 ```txt
 abc
 bac
-cba
-bca
 cab
 acb
+bca
+cba
 ```
 
 See the `test` folder for examples of input/output.
diff --git a/challenges/heap-algorithm/solutions/c/function/solution.c b/challenges/heap-algorithm/solutions/c/function/solution.c
index 8d5bafb..dd98d7b 100644
--- a/challenges/heap-algorithm/solutions/c/function/solution.c
+++ b/challenges/heap-algorithm/solutions/c/function/solution.c
@@ -5,34 +5,21 @@
 
 #include "input.h"
 
-char *swap(char *string, size_t index_from, size_t index_target) {
-  size_t string_length = strlen(string);
-  char *result = malloc(sizeof(char *) * (string_length));
-  for (size_t index = 0; index < string_length; index++) {
-    if (index == index_from) {
-      result[index] = string[index_target];
-    } else if (index == index_target) {
-      result[index] = string[index_from];
-    } else {
-      result[index] = string[index];
-    }
-  }
-  return result;
+void swap(char *string, size_t index_from, size_t index_target) {
+  char temporary = string[index_from];
+  string[index_from] = string[index_target];
+  string[index_target] = temporary;
 }
 
-void heap_algorithm(unsigned long number_of_elements_to_operate, char *string) {
+void heap_algorithm(char *string, size_t number_of_elements_to_operate) {
   if (number_of_elements_to_operate == 1) {
     printf("%s\n", string);
   } else {
-    heap_algorithm(number_of_elements_to_operate - 1, string);
+    heap_algorithm(string, number_of_elements_to_operate - 1);
     for (size_t index = 0; index < number_of_elements_to_operate - 1; index++) {
       bool is_even = number_of_elements_to_operate % 2 == 0;
-      if (!is_even) {
-        string = swap(string, index, number_of_elements_to_operate - 1);
-      } else {
-        string = swap(string, 0, number_of_elements_to_operate - 1);
-      }
-      heap_algorithm(number_of_elements_to_operate - 1, string);
+      swap(string, is_even ? index : 0, number_of_elements_to_operate - 1);
+      heap_algorithm(string, number_of_elements_to_operate - 1);
     }
   }
 }
@@ -40,6 +27,7 @@ void heap_algorithm(unsigned long number_of_elements_to_operate, char *string) {
 int main() {
   char *string = input();
   size_t string_length = strlen(string);
-  heap_algorithm(string_length, string);
+  heap_algorithm(string, string_length);
+  free(string);
   return EXIT_SUCCESS;
 }
diff --git a/challenges/heap-algorithm/test/1/output.txt b/challenges/heap-algorithm/test/1/output.txt
index 7895438..a8b686a 100644
--- a/challenges/heap-algorithm/test/1/output.txt
+++ b/challenges/heap-algorithm/test/1/output.txt
@@ -1,6 +1,6 @@
 abc
 bac
-cba
-bca
 cab
-acb
\ No newline at end of file
+acb
+bca
+cba
\ No newline at end of file
diff --git a/challenges/heap-algorithm/test/2/output.txt b/challenges/heap-algorithm/test/2/output.txt
index 4620edf..174b080 100644
--- a/challenges/heap-algorithm/test/2/output.txt
+++ b/challenges/heap-algorithm/test/2/output.txt
@@ -1,24 +1,24 @@
 cool
 ocol
-oocl
-oocl
 ocol
 cool
+oocl
+oocl
+loco
+olco
+cloo
+lcoo
+oclo
+colo
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+cloo
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-oolc
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-cool
-ocol
-oocl
-oocl
-ocol
-cool
-looc
-oloc
 oolc
-oolc
-oloc
-looc
\ No newline at end of file
+oolc
\ No newline at end of file
diff --git a/challenges/heap-algorithm/test/3/output.txt b/challenges/heap-algorithm/test/3/output.txt
index dcc6a8b..7310627 100644
--- a/challenges/heap-algorithm/test/3/output.txt
+++ b/challenges/heap-algorithm/test/3/output.txt
@@ -1,120 +1,120 @@
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\ No newline at end of file
+akyka
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\ No newline at end of file
diff --git a/challenges/heap-algorithm/test/4/output.txt b/challenges/heap-algorithm/test/4/output.txt
index d23ac2a..ec2f6fa 100644
--- a/challenges/heap-algorithm/test/4/output.txt
+++ b/challenges/heap-algorithm/test/4/output.txt
@@ -1,720 +1,720 @@
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\ No newline at end of file
+zetrya
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\ No newline at end of file
diff --git a/challenges/look-and-say-sequence-conway/solutions/c/function/character.c b/challenges/look-and-say-sequence-conway/solutions/c/function/character.c
index bd60265..3e8c0ff 100644
--- a/challenges/look-and-say-sequence-conway/solutions/c/function/character.c
+++ b/challenges/look-and-say-sequence-conway/solutions/c/function/character.c
@@ -5,10 +5,3 @@ void character_append(char* string, char character) {
   string[length] = character;
   string[length + 1] = '\0';
 }
-
-void character_append_many(char* string, char* characters) {
-  size_t characters_length = strlen(characters);
-  for (size_t index = 0; index < characters_length; index++) {
-    character_append(string, characters[index]);
-  }
-}
diff --git a/challenges/look-and-say-sequence-conway/solutions/c/function/character.h b/challenges/look-and-say-sequence-conway/solutions/c/function/character.h
index 1535477..e3a2037 100644
--- a/challenges/look-and-say-sequence-conway/solutions/c/function/character.h
+++ b/challenges/look-and-say-sequence-conway/solutions/c/function/character.h
@@ -13,13 +13,4 @@
  */
 void character_append(char* string, char character);
 
-/**
- * @brief Append many characters to a string, assuming string points to an array
- * with enough space.
- *
- * @param string
- * @param characters
- */
-void character_append_many(char* string, char* characters);
-
 #endif
diff --git a/challenges/look-and-say-sequence-conway/solutions/c/function/solution.c b/challenges/look-and-say-sequence-conway/solutions/c/function/solution.c
index 33bfb8a..85e19fb 100644
--- a/challenges/look-and-say-sequence-conway/solutions/c/function/solution.c
+++ b/challenges/look-and-say-sequence-conway/solutions/c/function/solution.c
@@ -1,3 +1,4 @@
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -5,22 +6,129 @@
 #include "character.h"
 #include "input.h"
 
+size_t string_get_length(const char* string) {
+  size_t length = 0;
+  while (string[length] != '\0') {
+    length++;
+  }
+  return length;
+}
+
+char* string_copy(const char* string) {
+  size_t source_length = string_get_length(string);
+  char* copy = malloc(sizeof(char) * (source_length + 1));
+  if (copy == NULL) {
+    perror("Error (string_copy)");
+    exit(EXIT_FAILURE);
+  }
+  size_t index;
+  for (index = 0; index < source_length; index++) {
+    copy[index] = string[index];
+  }
+  copy[index] = '\0';
+  return copy;
+}
+
+unsigned long long mathematics_absolute_value(const long long number) {
+  if (number >= 0) {
+    return number;
+  }
+  return -number;
+}
+
+void string_reverse(char* string) {
+  size_t string_length = string_get_length(string);
+  size_t index_start = 0;
+  size_t index_end = string_length - 1;
+  while (index_start < index_end) {
+    char temporary = string[index_start];
+    string[index_start] = string[index_end];
+    string[index_end] = temporary;
+    index_start++;
+    index_end--;
+  }
+}
+
+char* convert_character_to_string(const char character) {
+  char* string = malloc(sizeof(char) * 2);
+  if (string == NULL) {
+    perror("Error (convert_character_to_string)");
+    exit(EXIT_FAILURE);
+  }
+  string[0] = character;
+  string[1] = '\0';
+  return string;
+}
+
+char convert_digit_to_character(const char digit) {
+  return digit + '0';
+}
+
+void string_concatenate(char** destination, char* source) {
+  size_t destination_length = string_get_length(*destination);
+  size_t source_length = string_get_length(source);
+  size_t new_length = destination_length + source_length;
+  *destination = realloc(*destination, sizeof(char) * (new_length + 1));
+  if (*destination == NULL) {
+    perror("Error (string_concatenate)");
+    exit(EXIT_FAILURE);
+  }
+  size_t index_destination = destination_length;
+  for (size_t index_source = 0; index_source < source_length; index_source++) {
+    (*destination)[index_destination++] = source[index_source];
+  }
+  (*destination)[index_destination] = '\0';
+}
+
+char* convert_number_to_string(const long long integer) {
+  if (integer == 0) {
+    return convert_character_to_string('0');
+  }
+  bool is_negative = integer < 0;
+  size_t length = 1;
+  long long current = mathematics_absolute_value(integer);
+  while (current != 0) {
+    current = current / 10;
+    length++;
+  }
+  if (is_negative) {
+    length++;
+  }
+  char* string = malloc(sizeof(char) * length);
+  if (string == NULL) {
+    perror("Error (convert_number_to_string)");
+    exit(EXIT_FAILURE);
+  }
+  current = mathematics_absolute_value(integer);
+  size_t index = 0;
+  while (current != 0) {
+    string[index++] = convert_digit_to_character(current % 10);
+    current = current / 10;
+  }
+  if (is_negative) {
+    string[index++] = '-';
+  }
+  string[index] = '\0';
+  string_reverse(string);
+  return string;
+}
+
 int main() {
   char* string = input();
-  size_t string_length = strlen(string);
-  char* result = malloc(sizeof(char) * (string_length + 1));
+  size_t string_length = string_get_length(string);
+  char* result = string_copy("");
   for (size_t index = 0; index < string_length; index++) {
-    unsigned char number_of_appearances = 0;
+    long long number_of_appearances = 0;
     char value_to_search = string[index];
-    size_t iteration = index;
-    while (iteration < string_length && string[iteration] == value_to_search) {
+    while (number_of_appearances + index < string_length && string[number_of_appearances + index] == value_to_search) {
       number_of_appearances++;
-      iteration++;
     }
-    char* number_of_appearances_string = malloc(sizeof(char) * (string_length + 1));
-    snprintf(number_of_appearances_string, sizeof(result), "%hhu", number_of_appearances);
-    character_append_many(result, number_of_appearances_string);
-    character_append(result, value_to_search);
+    char* number_of_appearances_string = convert_number_to_string(number_of_appearances);
+    string_concatenate(&result, number_of_appearances_string);
+    char* value_string = convert_character_to_string(value_to_search);
+    string_concatenate(&result, value_string);
+    free(number_of_appearances_string);
+    free(value_string);
     index += number_of_appearances - 1;
   }
   printf("%s\n", result);
diff --git a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.c b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.c
index 54269aa..dad3ebb 100644
--- a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.c
+++ b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.c
@@ -13,48 +13,29 @@ void array_2D_int_print(int **array, size_t number_of_rows, size_t number_of_col
 }
 
 int **array_2D_int_input(size_t *number_of_rows, size_t *number_of_columns) {
-  int **array = malloc(sizeof(int) * 2);
-  *number_of_rows = 1;
-  *number_of_columns = 1;
-  array[0] = malloc(*number_of_columns * sizeof(int));
-  array[0][0] = 0;
-  char character;
-  size_t length = 1;
-  char *string = malloc(length * sizeof(char));
-  *string = '\0';
-  while ((character = getchar()) != EOF) {
-    if (character == '\n') {
-      int number = atoi(string);
-      array[*number_of_rows - 1][*number_of_columns - 1] = number;
-      length = 1;
-      memset(string, 0, length * sizeof(char));
-      *string = '\0';
-      *number_of_rows = *number_of_rows + 1;
-      *number_of_columns = 1;
-      array = realloc(array, *number_of_rows * sizeof(int *));
-      array[*number_of_rows - 1] = malloc(*number_of_columns * sizeof(int));
-    } else {
-      if (character == ' ') {
-        int number = atoi(string);
-        array[*number_of_rows - 1][*number_of_columns - 1] = number;
-        length = 1;
-        memset(string, 0, length * sizeof(char));
-        *string = '\0';
-        *number_of_columns = *number_of_columns + 1;
-      } else {
-        length++;
-        string = realloc(string, length * sizeof(char));
-        character_append(string, character);
-      }
+  *number_of_rows = 0;
+  *number_of_columns = 0;
+  int **array = malloc(sizeof(int *));
+  char *line = input();
+  while (string_get_length(line) != 0) {
+    char **integers_string = string_split(line, ' ', number_of_columns);
+    array[*number_of_rows] = malloc(*number_of_columns * sizeof(int));
+    for (size_t column = 0; column < *number_of_columns; column++) {
+      array[*number_of_rows][column] = atoi(integers_string[column]);
+      free(integers_string[column]);
     }
+    free(integers_string);
+    free(line);
+    line = input();
+    *number_of_rows += 1;
+    array = realloc(array, (*number_of_rows + 1) * sizeof(int *));
   }
-  int number = atoi(string);
-  array[*number_of_rows - 1][*number_of_columns - 1] = number;
+  free(line);
   return array;
 }
 
 int **array_2D_int_reverse_rows(int **array, size_t *number_of_rows, size_t *number_of_columns) {
-  int **rotated_array = malloc(*number_of_rows * sizeof(int));
+  int **rotated_array = malloc(*number_of_rows * sizeof(int *));
   for (size_t row = 0; row < *number_of_rows; row++) {
     rotated_array[row] = malloc(*number_of_columns * sizeof(int));
   }
diff --git a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.h b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.h
index 2b478c6..2b463a1 100644
--- a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.h
+++ b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/array_2D_int.h
@@ -6,6 +6,8 @@
 #include <string.h>
 
 #include "character.h"
+#include "input.h"
+#include "string.h"
 
 /**
  * @brief Prints a 2D array of integers.
diff --git a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/solution.c b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/solution.c
index db76433..62e9c91 100644
--- a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/solution.c
+++ b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/solution.c
@@ -9,6 +9,7 @@ int main() {
   size_t number_of_columns = 0;
   char *direction = input();
   int **array_input = array_2D_int_input(&number_of_rows, &number_of_columns);
+  size_t initial_number_of_rows = number_of_rows;
 
   int **array;
   if (strcmp(direction, "clockwise") == 0) {
@@ -19,6 +20,10 @@ int main() {
   array_2D_int_print(array, number_of_rows, number_of_columns);
 
   free(direction);
+  for (size_t row = 0; row < initial_number_of_rows; row++) {
+    free(array_input[row]);
+  }
+  free(array_input);
   for (size_t row = 0; row < number_of_rows; row++) {
     free(array[row]);
   }
diff --git a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/string.c b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/string.c
new file mode 100644
index 0000000..6a69e8b
--- /dev/null
+++ b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/string.c
@@ -0,0 +1,64 @@
+#include "string.h"
+
+size_t string_get_length(const char* string) {
+  size_t length = 0;
+  while (string[length] != '\0') {
+    length++;
+  }
+  return length;
+}
+
+char* string_copy(const char* string) {
+  size_t source_length = string_get_length(string);
+  char* copy = malloc(sizeof(char) * (source_length + 1));
+  if (copy == NULL) {
+    perror("Error (string_copy)");
+    exit(EXIT_FAILURE);
+  }
+  size_t index;
+  for (index = 0; index < source_length; index++) {
+    copy[index] = string[index];
+  }
+  copy[index] = '\0';
+  return copy;
+}
+
+char** string_split(const char* string, char separator, size_t* result_size) {
+  size_t string_length = string_get_length(string);
+  size_t index_string = 0;
+  size_t index_current = 0;
+  size_t index_result = 0;
+  char* current = malloc(sizeof(char) * (string_length + 1));
+  char** result = NULL;
+  if (current == NULL) {
+    perror("Error (string_split)");
+    exit(EXIT_FAILURE);
+  }
+  while (index_string < string_length) {
+    if (string[index_string] == separator) {
+      current[index_current] = '\0';
+      result = realloc(result, sizeof(char*) * (index_result + 1));
+      if (result == NULL) {
+        perror("Error (string_split)");
+        exit(EXIT_FAILURE);
+      }
+      result[index_result] = string_copy(current);
+      index_result++;
+      index_current = 0;
+    } else {
+      current[index_current] = string[index_string];
+      index_current++;
+    }
+    index_string++;
+  }
+  current[index_current] = '\0';
+  result = realloc(result, sizeof(char*) * (index_result + 1));
+  if (result == NULL) {
+    perror("Error (string_split)");
+    exit(EXIT_FAILURE);
+  }
+  result[index_result] = string_copy(current);
+  free(current);
+  *result_size = index_result + 1;
+  return result;
+}
diff --git a/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/string.h b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/string.h
new file mode 100644
index 0000000..fdf98b5
--- /dev/null
+++ b/challenges/rotate-2-dimensional-array-90-degrees/solutions/c/function/string.h
@@ -0,0 +1,35 @@
+#ifndef __STRING__
+#define __STRING__
+
+#include <errno.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+/**
+ * @brief Return the length of a string (excluding '\0').
+ *
+ * @param string
+ * @return size_t
+ */
+size_t string_get_length(const char* string);
+
+/**
+ * @brief Return the copy of a string.
+ *
+ * @param string
+ * @return string_t
+ */
+char* string_copy(const char* string);
+
+/**
+ * @brief Split a string into substrings using the specified separator and return them as an array and update the pointer `result_size` to the resulting size of the created array.
+ *
+ * @param string
+ * @param separator
+ * @param result_size
+ * @return string_t*
+ */
+char** string_split(const char* string, char separator, size_t* result_size);
+
+#endif