<pre><code>/**
 * @param {Element} root
 * @param {Element} target
 * @return {Elemnt | null}
 */
function previousLeftSibling(root, target) {
  const queue = [root];

  while (queue.length) {
    const len = queue.length;
    let prev = null;
    for (let i = 0; i &lt; len; i++) {
      const node = queue.shift();
      if (node === target) return prev;
      queue.push(...node.children);
      prev = node;
    }
  }

  return null;
}
</code></pre>

JS - BFS

Given a DOM tree and a target element, please return the previous left sibling. [https://cdn.bfe.dev/bfe/img/2a737BvrqUaK1F9YnxrD1JkYV58y8Le8_1063x546…