Add a swap function

sa.hpp

@@ -49,10 +49,17 @@ class solution {
 	}
 
 	void randomize() {
-		shuffle(items.begin(), items.end(), gen);
+		std::shuffle(items.begin(), items.end(), gen);
 
 		fitness = calculate_boxes();
 	}
+
+	void swap(solution &other) {
+		std::swap(fitness, other.fitness);
+		std::swap(items, other.items);
+		std::swap(capacity, other.capacity);
+		std::swap(gen, other.gen);
+	}
 };
 
 } // namespace sa