钉接龙回溯无限循环

Question

I'm making a peg solitaire resolver with backtracking in java.

This is the method I've done:

private void solve(Board board, ArrayList<Movement> solution, Boolean result) {
    ArrayList<Movement> movs = board.getMovements();
    for (Movement movement : movs) {
        if (board.isMovementValid(movement)) {
            board.doMovement(movement);
            if(!board.isSolution()) {
                solution.add(movement);
                solve(board, solution, result);
                result.setValue(false);
            } else {
                result.setValue(true);
            }

        }
    }
    result.setValue(false);
}

The problem is that I can't find the solution. Here is the output of the code: http://pastebin.com/raw.php?i=BhkLu3qr . As you can see the solution array is incomplete.

Thanks.

Answer 1

Not so elegant, but in order to track back and retry an alternative the step must be taken back:

ArrayList<Movement> movs = board.getMovements();
for (Movement movement : movs) {
    if (board.isMovementValid(movement)) {
        board.doMovement(movement);
        solution.add(movement);
        if(!board.isSolution()) {
            solve(board, solution, result);
            // Initialized to result.setValue(false);
            if (result.getValue()) { return; }
        } else {
            result.setValue(true);
            return;
        }
        solution.remove(movement);
        board.undoMovement(movement);
    }
}
result.setValue(false);

Also for a more general solution where you are satisfied with the first solution, I have added returns.

Answer 2

Assuming that your board.getMovements() method gives you a list of all possible moves from this point in the game, you're almost there. You just need to stop when you win. I've refactored at bit for clarity.

private boolean solve(Board board, ArrayList<Movement> solution) {
    // The base case: if it's already solved, we're done
    if (board.isSolution())
        return true;

    // Get all possible moves from this point
    ArrayList<Movement> movs = board.getMovements();
    for (Movement movement : movs) {
        if (board.isMovementValid(movement)) {
            board.doMovement(movement);
            solution.add(movement);
            if (solve(board, solution))
                // That move led to success :-)
                return true;
            else
                // That move led to failure :-(
                solution.remove(movement);
        }
    }
    return false;
}