i have thought a lot on this. but not getting O(n) solution. given n URL's and you have to find the unique URL in that (if there is any). you have to print that URL. thanks. O(n2) solution is a stupid solution, some O(n) is acheivable, but not getting how can it be O(n). i have lastly thoguht about tries. can it be like this ? some related to tries ? thanks alot in advance.