Use defined function for getting subtree size

file.hpp

@@ -53,12 +53,12 @@ public:
 		if(b == nullptr) return a;
 
 		if(a->priority < b->priority) {
-			a->size += b->size;
+			a->size += get_size(b);
 			a->right = join(a->right, b);
 			return a;
 		}
 		else {
-			b->size += a->size;
+			b->size += get_size(a);
 			b->left = join(a, b->left);
 			return b;
 		}