Многопоточное решение задачи о ферзях

Для решения задачи о ферзях на больших размерах доски я использую многопоточность. Но видимо я что-то неправильно делаю, потому что для любого размера доски выводится 0.

public class QueenThread {
    private int N;
    private int total;
    private Thread[] threads;
    private final Object o = new Object();

    public QueenThread(int N, int threads) {
        this.N = N;
        this.threads = new Thread[N];
    }
    public QueenThread(int N) {
        this.N = N;
        this.threads = new Thread[Runtime.getRuntime().availableProcessors()];
    }

    private class Worker implements Runnable {
        private int from;
        private int to;

        private Worker(int from, int to) {
            this.from = from;
            this.to = to;
        }

        @Override
        public void run() {
            for (int i = 0; i < threads.length; i++) {
                int[] board = new int[N + 1];
                solve(from, to, board);
            }
        }
    }

    public int calcQueen() throws InterruptedException {
        int threadsN = threads.length;
        for (int i = 0; i < threadsN; i++) {
            int from = i * N / threadsN + 1;
            int to = (i + 1) * N / threadsN;
            threads[i] = new Thread(new Worker(from, to));
            threads[i].start();
        }
        return total;
    }

    private void subSolve(int column, int[] board) {
        if (column > N) {
            synchronized (o) {
                total++;
            }
        }
        for (int j = 2; j <= N; j++) {
            if (canPut(j, column, board)) {
                board[column] = j;
                subSolve(column + 1, board);
            }
        }
    }
    private void solve(int start, int end, int[] board) {
        for (int i = start; i <= end; i++) {
            board[1] = i;
            subSolve(2, board);
        }
    }
    private boolean canPut(int row, int column, int[] board) {
        for (int i = 1; i <= column; i++) {
            if (row == board[i] || board[i] - row == column - i || board[i] - row == i - column) { return false; }
        }
        return true;
    }

    public static void main(String[] args) throws InterruptedException {
        QueenThread a = new QueenThread(10);
        System.out.println(a.calcQueen());
    }
}