尝试实现极小极大算法时出现 java.lang.OutOfMemoryError

Question

我在 Java 中实现了野生井字棋变体的极小极大算法，并且偶然发现了一个问题。我有一个 Node 类，它保存游戏网格和作为其子项的 Node 对象的 ArrayList，以及一个递归实现该算法的 minimax 方法。

我得到的错误是:

Exception in thread "main" java.lang.OutOfMemoryError: GC overhead limit exceeded
        at Grid.<init>(Grid.java:35)
        at MiniMax$Node.findChildren(MiniMax.java:27)
        at MiniMax.minimax(MiniMax.java:135)
        at MiniMax.minimax(MiniMax.java:126)
        at MiniMax.minimax(MiniMax.java:139)
        at MiniMax.minimax(MiniMax.java:126)
        at MiniMax.minimax(MiniMax.java:139)
        at MiniMax.minimax(MiniMax.java:126)
        at MiniMax.minimax(MiniMax.java:139)
        at MiniMax.nextMove(MiniMax.java:77)
        at ComputerPlayer.play(ComputerPlayer.java:12)
        at TicTacToe.main(TicTacToe.java:146)

Process finished with exit code 1

我认为出现该问题的原因是每次都递归创建并存储到 ArrayList 中的子级数量较多(节点总数:2^8 * 8!)。

这是 Node 类:

 private static class Node
    {
        protected Grid grid;
        protected ArrayList<Node> children;

        public Node(Grid grid)
        {
            this.grid = grid;
            children = new ArrayList<>();
        }

        //Find all possible next moves
        public void findChildren()
        {
            char[][] board = grid.getGrid();
            for(int i = 0; i < board.length; i++)
            {
                for(int j = 0; j < board.length; j++)
                {
                    if(board[i][j] == ' ')
                    {
                        board[i][j] = 'X';
                        children.add(new Node(new Grid(board)));
                        board[i][j] = 'O';
                        children.add( new Node(new Grid(board)));
                        board[i][j] = ' ';
                    }
                }
            }
        }
    }

这是极小极大的实现:

private int minimax(Node state, int depth, boolean isMaximizer)
    {
        //If the game is in a terminal state or has reached the desired depth
        boolean someoneWon = state.grid.someoneHasWon();
        boolean isDraw =  state.grid.isDraw();
        if(someoneWon || isDraw || depth == 3)
        {
            return evaluateState(someoneWon, isDraw, !isMaximizer);//Evaluate the state
        }
        //MAX player's turn
        if(isMaximizer)
        {
            //Find maximum score of all possible state's scores
            int bestScore = Integer.MIN_VALUE;
            state.findChildren();
            for(int i = 0; i < state.children.size(); i++)
            {
                Node child = state.children.get(i);
                int score = minimax(child, depth + 1, false);
                bestScore = Math.max(bestScore, score);
            }
            return bestScore;
        }
        else//MIN player's turn
        {
            //Find minimum score of all possible move's scores
            int bestScore = Integer.MAX_VALUE;
            state.findChildren();
            for(int i = 0; i < state.children.size(); i++)
            {
                Node child = state.children.get(i);
                int score = minimax(child, depth + 1, true);
                bestScore = Math.min(bestScore, score);
            }
            return bestScore;
        }
    }

Answer 1

不要创建子节点列表，而是将迭代移至 Node (或等效项)。您会注意到，您不需要每次都创建一个新板 - 只需替换您的状态即可完成后更改。