given two graphs and check whether graphs are isomorphic or not

Checking for a graph might be hard …checking two trees for isomorphic or not is easy.

It is because it would be hard to select a starting pt in the graph, it is easy as there is a root in a tree.

But still you can try selecting random starting pts in two graphs. There would be n^2 starting pairs ans then O(n) complexity for each traverse. Hence the total complexity maybe O(n^3).