Shahrzad, While your algorithm is correct it is very inefficient. The full array is scanned for every element in the array. If the array contains 1,000 elements, 1,000,000 comparisons will be performed and mktree_array() will be called 1,000 times. This is known as order n squared: O(n^2) and is about as slow as it gets. Oh, I see that you "have to use [a] recursive function." Is this is a homework assignment? If not and you want to make it faster, you can speed up the code by creating a new array first that maps from $a['nid'] => $a (the array element) to create a hash table. Then loop over the array again and use the hash table to find the parent of each element ($a) and add it to $a. This will be much faster without being any more complex (exchange recursion for two simple loops). Peace, David
