Question

I am working on the alpha-beta pruning algorithm with Tic-Tac-Toe game (3×3). Currently for any given instance of the 3×3 grid, I was able to figure out the best case:

public Best chooseAlphaBetaMove(int whosMov, int alpha, int beta) {

    Best reply = new Best();  
    Best myBest = new Best();

    if ((scoreGrid()==COMPUTER_WIN) || (scoreGrid()==OPPONENT_WIN) || 
                                       (scoreGrid()==GAME_DRAW)) {
        int score = scoreGrid();
        return new Best(score,-3,-3,count);
    }

    if (whosMov==COMPUTER_MOVE) {
        myBest.score = alpha;
    } else {
        myBest.score = beta;
    }

    for (int i=0; i<3; i++) {
    for (int j=0; j<3; j++) {
        if (layOut[i][j]==0) {
            moveGrid(whosMov,i,j);
            reply = chooseAlphaBetaMove(-whosMov,alpha,beta); 
            unmoveGrid(i,j);

            if ((whosMov==COMPUTER_MOVE)&&(reply.score>myBest.score)) {
                myBest.score = reply.score;
                alpha = reply.score;
                myBest.row = i;
                myBest.column = j;
            }

            if  ((whosMov==OPPONENT_MOVE)&&(reply.score<myBest.score)) {
                myBest.score = reply.score;
                beta = reply.score;
                myBest.row = i;
                myBest.column = j;
            }
            if (beta <= alpha) return myBest;
        }
    }
    }

        return myBest;
}

Where the Best structure is:

public class Best {

    public int score;
    public int row;
    public int column;
    public int count
}

Given an initial grid and who will move next, I can know the best score and the best position for this next player to go. However, I can’t figure out how to print the whole path for this best move. (Note – I don’t want the whole search path. I only want to print out the single search path starting from this best move till the leaf). Any thoughts? thanks!

Answer 1

As you go recursively down each path, you need to keep track of it, probably by passing a reference to a List/Stack into each chooseAlphaBetaMove call. When you find a better path than the current best path, you take a copy of the current path, and store it as the “best path so far”. Once you have finished, you can then print out the best path.