计算$D=\left|\begin{array}{cccccc}
1 & 2 & 3 & \cdots & n-1 & n \\
x & 1 & 2 & \cdots & n-2 & n-1 \\
x & x & 1 & \cdots & n-3 & n-2 \\
\vdots & \vdots & \vdots & & \vdots & \vdots \\
x & x & x & \cdots & 1 & 2 \\
x & x & x & \cdots & x & 1
\end{array}\right|$