计算 $\left|\begin{array}{ccccccc}
1 & 1 & 0 & \cdots & 0 & 0 & 0 \\
1 & 1 & 1 & \cdots & 0 & 0 & 0 \\
0 & 1 & 1 & \cdots & 0 & 0 & 0 \\
& \ldots \ldots & \ldots & \ldots & \ldots & \cdots & \\
0 & 0 & 0 & \cdots & 1 & 1 & 1 \\
0 & 0 & 0 & \cdots & 0 & 1 & 1
\end{array}\right|$