feat(solutions): add a-phone-code/c/function

challenges/a-phone-code/solutions/c/function/character.h

@@ -0,0 +1,16 @@
+#ifndef __CHARACTER__
+#define __CHARACTER__
+
+#include <stdlib.h>
+#include <string.h>
+
+/**
+ * @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