update best answer function

applications/ColossalChat/coati/reasoner/guided_search/mcts.py

@@ -120,17 +120,16 @@ class MCTS(BaseModel):
             self.back_propagation(child)
 
         return self.get_best_answer()
-
-    def get_best_answer(self):
+    
+    def _iter_nodes(self):
         to_visit = deque([self.root])
-        best_node = self.root
-
         while to_visit:
             current_node = to_visit.popleft()
-            if current_node.Q > best_node.Q:
-                best_node = current_node
+            yield current_node
             to_visit.extend(current_node.children)
-
+    
+    def get_best_answer(self):
+        best_node = max(self._iter_nodes(), key=lambda node: node.Q, default=self.root)
         return best_node.answer
 
     def self_refine(self, node: MCTSNode):