added stack ADT

stack.c

@@ -0,0 +1,52 @@
+#include "stack.h"
+
+/* 
+ * [item]-->[item]-->[item]-->[item]-->NULL
+ * ^ top                       ^bottom
+ */
+
+Stack *stack_new(void) {
+        return NULL;
+}
+
+int stack_isempty(Stack *s) {
+        return (s == NULL);
+}
+
+void *stack_top(Stack *s) {
+        if (stack_isempty(s))
+                return NULL;
+        return s->item;
+}
+
+void stack_free(Stack *s) {
+        while (!stack_isempty(s))
+                stack_pop(&s);
+}
+
+/* Push new item onto stack */
+void stack_push(Stack **s, void *item) {
+        Stack *top;
+
+        top = malloc(sizeof(Stack));
+        if (top == NULL)
+                return;
+        top->item = item;
+        top->next = *s;
+        *s = top;
+}
+
+/* Pop stack and return item */
+void *stack_pop(Stack **s) {
+        Stack *next;
+        void *item;
+
+        if (*s == NULL)
+                return NULL;
+        next = (*s)->next;
+        item = (*s)->item;
+        free(*s);
+        *s = next;
+
+        return item;
+}

stack.h

@@ -0,0 +1,14 @@
+#include <stdlib.h>
+
+struct Stack {
+        struct Stack *next;
+        void *item;
+};
+typedef struct Stack Stack;
+
+Stack *stack_new(void);
+int stack_isempty(Stack *s);
+void *stack_top(Stack *s);
+void stack_push(Stack **s, void *item);
+void *stack_pop(Stack **s);
+void stack_free(Stack *s);