NQueens.java

class NQueens {
    public int totalNQueens(int n) {
        int[] B = new int[n];
        return nqueens(B, 0);
    }

    public int nqueens(int[] B, int row) {
        if (row == B.length) return 1; 
        int count = 0;
        for (int j=0; j<B.length; j++) {
            if (!check(B, row, j)) continue;
            B[row] = j;
            count += nqueens(B, row+1);
        }
        return count;
    }

    public boolean check(int[] B, int row, int col) {
        for (int i=0; i < row; i++) {
            // check same column
            if (B[i] == col) return false;
            // check diagonal
            if ((row + col == B[i] + i) || (row - col == i - B[i]))
                return false;
        }
        return true;
    }
}