First try, traverse tree and check

Another approach is to convert the tree to string using pre-order and
in-order, then to check the substring, this approach's time CP is
O(m+n), where m and n are node in each trees; the space CP is O(m+n),
which is big for this problem. This commit's approach's time CP is
O(mn), the tighter bound is O(m+nk), the space CP improve a lot, which
is O(log(m)+log(n))