用于N皇后的Java回溯程序: StackOverFlow错误

public class NQueens {
    private int[] queens; // 用于存储每一行皇后的列位置
    private int count; // 解的数量

    public int solveNQueens(int n) {
        queens = new int[n];
        count = 0;
        backtrack(0, n);
        return count;
    }

    private void backtrack(int row, int n) {
        if (row == n) {
            count++;
            return;
        }
        for (int col = 0; col < n; col++) {
            if (isValid(row, col)) {
                queens[row] = col;
                backtrack(row + 1, n);
            }
        }
    }

    private boolean isValid(int row, int col) {
        for (int i = 0; i < row; i++) {
            if (queens[i] == col || Math.abs(queens[i] - col) == row - i) {
                return false;
            }
        }
        return true;
    }
}