feat(solutions): add heap-algorithm/c/function

challenges/heap-algorithm/solutions/c/function/README.md

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+# heap-algorithm/c/function
+
+Created by [@Divlo](https://github.com/Divlo) on 8 November 2021.

challenges/heap-algorithm/solutions/c/function/character.c

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+#include "character.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+void character_append(char* string, char character) {
+  size_t length = strlen(string);
+  string[length] = character;
+  string[length + 1] = '\0';
+}

challenges/heap-algorithm/solutions/c/function/character.h

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+#ifndef __CHARACTER__
+#define __CHARACTER__
+
+/**
+ * @brief Append a character to a string, assuming string points to an array
+ * with enough space.
+ *
+ * @param string
+ * @param character
+ */
+void character_append(char* string, char character);
+
+#endif

challenges/heap-algorithm/solutions/c/function/input.c

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+#include "input.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "character.h"
+
+char* input() {
+  char character;
+  size_t length = 1;
+  char* string = malloc(length * sizeof(char));
+  *string = '\0';
+  while ((character = getchar()) != '\n' && character != EOF) {
+    length++;
+    string = realloc(string, length * sizeof(char));
+    character_append(string, character);
+  }
+  return string;
+}

challenges/heap-algorithm/solutions/c/function/input.h

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+#ifndef __INPUT__
+#define __INPUT__
+
+/**
+ * @brief Read a line from stdin.
+ *
+ * @return char*
+ */
+char* input();
+
+#endif

challenges/heap-algorithm/solutions/c/function/solution.c

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+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#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 heap_algorithm(unsigned long number_of_elements_to_operate, char *string) {
+  if (number_of_elements_to_operate == 1) {
+    printf("%s\n", string);
+  } else {
+    heap_algorithm(number_of_elements_to_operate - 1, string);
+    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);
+    }
+  }
+}
+
+int main() {
+  char *string = input();
+  size_t string_length = strlen(string);
+  heap_algorithm(string_length, string);
+  return EXIT_SUCCESS;
+}