def expand_leaf(node, state):
""" Adds a new leaf to the tree by creating a new child node for the given node.
Args:
node: The node for which a child will be added.
state: The state of the game.
Returns: The added child node.
"""
try:
action = node.untried_actions.pop()
except IndexError:
return node
new_node = MCTSNode(parent=node, parent_action=action)
new_node.state = fun_board.next_state(node.state, action)
new_node.untried_actions = fun_board.legal_actions(new_node.state)
node.child_nodes[action] = new_node
return new_node
