feat: expand and child node

algorithms/a_star.py

@@ -45,11 +45,16 @@ class Node:
         return hash(self.state)
 
 
-def child_node(action: Action) -> Node:
-    pass
+def expand(node: Node) -> List[Node]:
+    return [child_node(node=node, action=action) for action in actions(node.state)]
 
 
-def actions(state: State) -> List[str]:
+def child_node(node: Node, action: Action) -> Node:
+    next_state = result(state=node.state, action=action)
+    return Node(state=next_state, parent=node, action=action)
+
+
+def actions(state: State) -> List[Action]:
     pass