Implement stack functions

stack.c

@@ -1,33 +1,60 @@
 #include <stdlib.h>
 #include "stack.h"
 
-//TODO: grundlegende Stackfunktionen implementieren:
-/*  * `push`: legt ein Element oben auf den Stack,
-    * `pop`: entfernt das oberste Element,
-    * `top`: liefert das oberste Element zurück,
-    * `clearStack`: gibt den gesamten Speicher frei. */
-
 // Pushes data as pointer onto the stack.
 StackNode *push(StackNode *stack, void *data)
 {
+    StackNode *newNode = (StackNode *)malloc(sizeof(StackNode));
 
+    if (newNode == NULL)
+    {
+        return stack;
+    }
+
+    newNode->data = data;
+    newNode->next = stack;
+
+    return newNode;
 }
 
-// Deletes the top element of the stack (latest added element) and releases its memory. (Pointer to data has to be
-// freed by caller.)
+// Deletes the top element of the stack (latest added element) and releases its memory.
+// Pointer to data has to be freed by caller.
 StackNode *pop(StackNode *stack)
 {
+    StackNode *nextNode = NULL;
 
+    if (stack != NULL)
+    {
+        nextNode = stack->next;
+        free(stack);
+    }
+
+    return nextNode;
 }
 
 // Returns the data of the top element.
 void *top(StackNode *stack)
 {
+    void *data = NULL;
 
+    if (stack != NULL)
+    {
+        data = stack->data;
+    }
+
+    return data;
 }
 
 // Clears stack and releases all memory.
 void clearStack(StackNode *stack)
 {
+    StackNode *currentNode = stack;
+    StackNode *nextNode = NULL;
 
+    while (currentNode != NULL)
+    {
+        nextNode = currentNode->next;
+        free(currentNode);
+        currentNode = nextNode;
+    }
 }

stack.h

@@ -7,13 +7,22 @@ The latest element is taken from the stack. */
 
 #include <stdlib.h>
 
-//TODO: passenden Datentyp als struct anlegen
+/*
+ * One node of the stack.
+ * data stores a generic pointer.
+ * next points to the next element below this one.
+ */
+typedef struct StackNode
+{
+    void *data;
+    struct StackNode *next;
+} StackNode;
 
 // Pushes data as pointer onto the stack.
 StackNode *push(StackNode *stack, void *data);
 
-// Deletes the top element of the stack (latest added element) and releases its memory. (Pointer to data has to be
-// freed by caller.)
+// Deletes the top element of the stack (latest added element) and releases its memory.
+// Pointer to data has to be freed by caller.
 StackNode *pop(StackNode *stack);
 
 // Returns the data of the top element.