Hi all! I have been caught in this problem from past few hours....my thoughts are in following way:- first we'll do DFS and store number of nodes in the subtree rooted at every node..after this we would take all these numbers in sorted order and iterate from both sides(start and end) and print each pair till we reach middle..but this won't work if more than 2 subtree has same number of nodes.....please help here... or please discuss if there is any other simpler way than this..thanks in advance!! [CUT] EDIT: The problem is solved. From here it was nothing but to just take an "OR" of all combinations possible from each node with overall array of possible combinations.