added is_empty function; fixes #1

data-structures/stack/main.cpp

@@ -6,6 +6,7 @@ int main(int argc, char const *argv[]) {
   int k;
   stack *s = NULL;
   init_stack(&s);
+  std::cout << "its empty " << is_empty(&s) << std::endl;
   push(&s,2);
   push(&s,4);
   push(&s,5);
@@ -25,6 +26,7 @@ int main(int argc, char const *argv[]) {
   push(&s,37);
 
 
+  std::cout << "its empty " << is_empty(&s) << std::endl;
   k = pop(&s);
   printf("%d %d\n",k,s->size );
   k = pop(&s);

data-structures/stack/stack.cpp

@@ -62,4 +62,8 @@ void free_stack(stack **s){
   }
 }
 
+bool is_empty(stack **s){
+  return (*s)->size == 0 ? true : false;
+}
+
 #endif

data-structures/stack/stack.h

@@ -18,4 +18,5 @@ void push(stack **s, int key);
 int pop(stack **s);
 void print(stack **s);
 void free_stack(stack **s);
+bool is_empty(stack **s);
 #endif