feat(solutions): add reverse-polish-notation/c/function

challenges/reverse-polish-notation/solutions/c/function/README.md

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+# reverse-polish-notation/c/function
+
+Created by [@Divlo](https://github.com/Divlo) on 12 October 2021.

challenges/reverse-polish-notation/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/reverse-polish-notation/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/reverse-polish-notation/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/reverse-polish-notation/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/reverse-polish-notation/solutions/c/function/solution.c

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+#include <ctype.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "input.h"
+#include "stack.h"
+
+bool is_integer(char* string) {
+  size_t string_length = strlen(string);
+  for (size_t index = 0; index < string_length; index++) {
+    if (!isdigit(string[index])) {
+      return false;
+    }
+  }
+  return true;
+}
+
+int main() {
+  char* string = input();
+  struct Stack* stack = stack_initialization();
+  char* token = strtok(string, " ");
+  while (token != NULL) {
+    if (is_integer(token)) {
+      intptr_t number;
+      sscanf(token, "%ld", &number);
+      stack_push(stack, (void*)number);
+    } else {
+      intptr_t number1 = (intptr_t)stack_pop(stack);
+      intptr_t number2 = (intptr_t)stack_pop(stack);
+      intptr_t result = 0;
+      if (strcmp(token, "+") == 0) {
+        result = number2 + number1;
+      } else if (strcmp(token, "-") == 0) {
+        result = number2 - number1;
+      } else if (strcmp(token, "*") == 0) {
+        result = number2 * number1;
+      } else {
+        result = number2 / number1;
+      }
+      stack_push(stack, (void*)result);
+    }
+    token = strtok(NULL, " ");
+  }
+  printf("%ld\n", (intptr_t)stack_pop(stack));
+  return EXIT_SUCCESS;
+}

challenges/reverse-polish-notation/solutions/c/function/stack.c

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+#include "stack.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+struct Stack *stack_initialization() {
+  struct Stack *stack = malloc(sizeof(*stack));
+  if (stack == NULL) {
+    exit(EXIT_FAILURE);
+  }
+  stack->first = NULL;
+  stack->length = 0;
+  return stack;
+}
+
+void stack_push(struct Stack *stack, void *data) {
+  struct Node *node_new = malloc(sizeof(*node_new));
+  if (stack == NULL || data == NULL) {
+    exit(EXIT_FAILURE);
+  }
+  node_new->data = data;
+  node_new->next = stack->first;
+  stack->first = node_new;
+  stack->length = stack->length + 1;
+}
+
+void *stack_pop(struct Stack *stack) {
+  if (stack == NULL) {
+    exit(EXIT_FAILURE);
+  }
+  struct Node *node = stack->first;
+  void *data = NULL;
+  if (node != NULL) {
+    stack->first = node->next;
+    data = node->data;
+    free(node);
+  }
+  stack->length = stack->length - 1;
+  return data;
+}

challenges/reverse-polish-notation/solutions/c/function/stack.h

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+#ifndef __STACK__
+#define __STACK__
+
+#include <stdlib.h>
+
+// LIFO = Last In First Out
+struct Stack {
+  struct Node *first;
+  size_t length;
+};
+
+struct Node {
+  void *data;
+  struct Node *next;
+};
+
+struct Stack *stack_initialization();
+
+void stack_push(struct Stack *stack, void *data);
+
+void *stack_pop(struct Stack *stack);
+
+#endif