  Bulk-edge correspondence is one of the most distinct properties of
topological insulators. In particular, the 1D winding number $\n$ has a
one-to-one correspondence to the number of edge states in a chain of
topological insulators with boundaries. By properly choosing the unit cells, we
carry out numerical calculation to show explicitly in the extended SSH model
that the winding numbers corresponding to the left and right unit cells may be
used to predict the numbers of edge states on the two boundaries in a finite
chain. Moreover, by drawing analogy between the SSH model and QWZ model, we
show that the extended SSH model may be generalized to the extended QWZ model.
By integrating the ``magnetic field'' over the momentum strip $0\le p_2 \le
\pi, 0\le p_1 2\pi$ in the Brillouin zone, we show a identity relating the 2D
Chern number and the difference between the 1D winding numbers at $p_2=0 $ and
$p_2 =\pi$.
