Tree iteration without recursion - php

Question

So, I was trying to print my class:
class category {
public $name;
public $id;
public $subCats = array();
public function __construct($name = "", $id = "") {
$this->name = $name;
$this->id = $id;
}
public function add_sub_cat($subCat) {
array_push($this->subCats, $subCat);
}
}
In two ways recursive and iterative, first one I did without any problems:
function recursive_print($category, $path) {
?>
<li><div id="category-name" ><p><? echo $category->name ?></p>
</div></li>
<?php foreach($category->subCats as $subCat) { ?>
<ul>
<?php recursive_print($subCat, $path.=$subCat->id) ?>
</ul>
<?php }
}
But now I got stuck on second part of this task.
Do I have to modify my class?
Is it even possible to print without recursion?
I have read this but it did not cleared anything.
Maybe someone have better tutorial or any advice?

Answer 1

Walking a tree without recursion is always an interesting problem.
The basic idea is you need to keep track of a stack by yourself. (Function calls are implemented by pushing temporary variables and a return address to a stack before the call, and popping that return address afterwards, so when you make a recursive function, you're avoiding having to do this yourself.)
Here's a non-recursive implementation:
function tree_print($root_category) {
$stack = array(array($root_category));
echo '<ul>'."\n";
while (count($stack)) {
$category = array_shift($stack[count($stack)-1]);
echo '<li><div id="category-name"><p>' . $category->name . '</p></div></li>'."\n";
echo '<ul>'."\n";
$stack[] = $category->subCats;
while (count($stack) && count($stack[count($stack)-1]) == 0) {
echo '</ul>'."\n";
array_pop($stack);
}
}
}
In each iteration round the main loop, we shift the first tree node from the array at the top of the stack, print it, open a <ul> tag, and push to the stack an array of all its child nodes. Then, we get rid of any empty arrays from the top of the stack, closing one <ul> tag for each such empty array.
See it running here: https://3v4l.org/umpvf