* Added `stack_init()` function

* Added `DEFAULT_CAPACITY` constant 

stack.c

 #include "stack.h"
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define DEFAULT_CAPACITY 4
+
+void stack_init(stack *s)
+{
+    s->top = -1;
+    s->capacity = DEFAULT_CAPACITY;
+    s->data = malloc(sizeof(int) * DEFAULT_CAPACITY);
+}
\ No newline at end of file

stack.h

@@ -7,4 +7,6 @@ typedef struct _stack {
     int top;
 } stack;
 
+void stack_init(stack *stack);
+
 #endif