TADM2E 5.17

use dfs to find cycle of lenght 3.

we maintain an array parent[i].

while processing any backedge u-v we check whether gradparent of u is equal to v .if true we found a cycle of lenght 3. gradparentU = parent[parent[u]]

i,e gradparentU == v,then found cycle of lenght 3.stop futher exploration of dfs