for no. of different binary tree---
for n nodes the formula is 2^n-n in this formula value of
nodes are not considered, only the way nodes can be present
in a tree so for 3 nodes there can be 5 ways, for each way
no. of possibilty of values is 3! so answer comes out to be
5*6=30

Q # 1 : in which graph algorithm do we start finding
vertices that should be first in the topological order and
then apploy the fact that every vertex must come before its
successors in the topolgical order.