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Showing posts from October 31, 2009

Combined Pre-Order and Post-Order Traversal: A Stackless Approach

Here's a slight mod to my previous code which gets rid of the stack altogether. It just relies on the parentNode link present in all nodes in a DOM tree. Performance is comparable. Memory usage is less - of course, we don't have a stack anymore.

public function traverse($root) { // Start at document root $_flag=FALSE; $_node=$root->documentElement; // Repeat until we reach the top while ($_node) { if (!$_flag) { /* Insert pre-order sequence here */ if ($_node->firstChild) { // Descend to branch $_flag=FALSE; $_node=$_node->firstChild; continue; } } /* Insert post-order sequence here */ // Reset flag $_flag=FALSE; if ($_node->nextSibling) $_node=$_node->nextSibling; else { // Ascend to parent node $_flag=TRUE; $_node=$_node->parentNode; } } }
Like it's predecessor, it doesn't need a "visited" flag attached to its nodes. The $_flag variable here is key to how it works. It is a simple t…

Combined Pre-order and Post-order Non-recursive DOM Tree Traversal Algorithm

After a lot of googling around, I discovered that a lot of sites discuss pre-order, in-order and post-order algorithms focused on binary trees. Most of the code are familiar textbook materials. Only a few take up more complicated tree structures like DOM documents.

Both recursive and iterative approaches to tree traversal have their pros and cons. And I'm not here to overemphasize any of these. Instead we'll take a different approach that's as fast, if not better than any other. The algorithm presented here is written in PHP and allows you to combine both pre-order and post-order traversal sequence in a single pass/loop.

We'll be needing a stack that will contain all the nodes we passed along the way as we go through each branch. The traditional method of iterative traversal using a "visited" flag attached to each node has a 1:1 correspondence to the total number of nodes. On the other hand, the use of a stack is proportional to the height (or depth) of the t…