lowestHidingElement

/**
 * Find the deepest DOM element that contains all given elements.
 *
 * =========================================================
 * Data Structure
 * =========================================================
 * - DOM Tree (N-ary tree)
 *
 * =========================================================
 * Core Idea
 * =========================================================
 * This is essentially a Lowest Common Ancestor (LCA) problem.
 *
 * We start from the first element and walk upward through
 * its ancestors until we find the first ancestor that
 * contains every element in the input array.
 *
 * Since we move from deepest -> root,
 * the first valid ancestor is the answer.
 *
 * =========================================================
 * Example
 * =========================================================
 *
 * <section>
 *   <button>Save</button>
 *   <button>Cancel</button>
 * </section>
 *
 * Start from:
 * - Save button
 *
 * Check:
 * - saveButton.contains(cancelButton) => false
 * - section.contains(saveButton) => true
 * - section.contains(cancelButton) => true
 *
 * Therefore:
 * => section is the lowest hiding element.
 *
 * =========================================================
 * Time Complexity
 * =========================================================
 * O(h * n)
 *
 * h = height of DOM tree
 * n = number of elements
 *
 * =========================================================
 * Space Complexity
 * =========================================================
 * O(1)
 *
 * @param {Element[]} elements
 * @returns {Element | null}
 */
export default function lowestHidingElement(elements) {
  // Start from the first element itself.
  let current = elements[0];

  // Walk upward toward the root.
  while (current) {
    // Check whether current ancestor contains all elements.
    const containsAll = elements.every((element) =>
      current.contains(element),
    );

    // First matching ancestor is the deepest valid answer.
    if (containsAll) {
      return current;
    }

    current = current.parentElement;
  }

  return null;
}