21Implement getElementsByClassName() recursively

Implement getElementsByClassName(root, classNames) without using the built-in getElementsByClassName or querySelectorAll. Walk the DOM tree recursively and return an array of every descendant element of root that has all of the given classes.

// <div id="root">
//   <div class="card featured">…<span class="card"></span></div>
//   <p class="card"></p>
// </div>
getElementsByClassName(root, "card"); // [div.card.featured, span.card, p.card]
getElementsByClassName(root, "featured card"); // [div.card.featured]

Requirements

• classNames may contain several names separated by spaces; an element must have all of them, in any order.
• Match whole class names only: "item" does not match class="items".
• Return elements in document order (depth-first, parent before children).
• Do not include root itself.
• An empty or blank classNames returns [].