making a undirected tree
v [ 1 ] . push_back ( 2 ) ; v [ 2 ] . push_back ( 1 ) ; v [ 1 ] . push_back ( 3 ) ; v [ 3 ] . push_back ( 1 ) ; v [ 3 ] . push_back ( 4 ) ; v [ 4 ] . push_back ( 3 ) ; v [ 3 ] . push_back ( 5 ) ; v [ 5 ] . push_back ( 3 ) ;