fix compilation error

Assignment_3/lib/ADT_Stack.h

@@ -1,3 +1,6 @@
+#ifndef ADT_Stack_H
+#define ADT_Stack_H
+
 #include <stack>
 #include <vector>
 #include <mutex>
@@ -31,3 +34,5 @@ class ADTOperationQueue{
     operation dequeue();
     size_t size();
 };
+
+#endif

Assignment_3/lib/Stack.cpp

@@ -12,7 +12,7 @@ Stack::Stack(ADTOperationQueue * queue){
 
 void Stack::push(int element) { return; }
 int Stack::pop() { return 0; }
-void Stack::size() { return 0; }
+int Stack::size() { return 0; }
 
 std::string Stack::print_stack(){
     Node* c = this->first->next;

Assignment_3/main.cpp

@@ -41,6 +41,7 @@ std::vector<operation> generate_operations_uniform(){
                 break;
         }
     }
+    return operations;
 }
 
 void run_worker(std::vector<operation>* operations, Stack* stack) {