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<?php class CategoryTree { protected $roots = []; public function addCategory($category, $parent) { if ($this->categoryExists($this->roots, $category)) { throw new InvalidArgumentException("Category: $category already exists."); } if ($parent === null) { $this->roots[$category] = []; return; } $node = &$this->findNode($parent, $this->roots); if(!isset($node)) { throw new InvalidArgumentException("Parent node: $parent not found."); } $node[$category] = []; } public function getChildren($parent) { $node = $this->findNode($parent, $this->roots); if(!isset($node)) { throw new InvalidArgumentException("Parent node: $parent not found."); } return array_keys($node); } protected function &findNode($node, &$root) { if (in_array($node, array_keys($root))) { return $root[$node]; } foreach ($root as $n) { return $this->findNode($node, $n); } } protected function categoryExists($nodes, $category) { $exists = false; foreach($nodes as $key => $node) { if (in_array($category, $this->getChildren($key))) { return true; } $exists = $this->categoryExists($node, $category); } return $exists; } } // For testing purposes (do not submit uncommented): $c = new CategoryTree; $c->addCategory('A', null); $c->addCategory('B', 'A'); $c->addCategory('A', 'A'); echo implode(',', $c->getChildren('A'));