consider a binary tree in which 1,2,3,4,5,6,7 are inserted in level order.Then node 7 will be the last one to get inserted. Now we want to find the worst case search time to find 7. Consider this BFS order 1,2,3,4,5,6,7 in which we need to traverse all the 7 nodes to reach 7 i.e O(n). And the best case BFS order to find 7 is is 1,3,2,7,6,4,5 I hope this helps!