<pre class="shiki github-dark shaku"><code>/** * @param {Element} root * @param {Element} target * @return {Elemnt | null} */function previousLeftSibling(root, target) { // level order traversal let queue = [root]; while(queue.length) { const copy = queue.slice(); queue = []; for(let i=0; i&lt;copy.length; i++) { // current level if(copy[i].isEqualNode(target)) { return copy[i-1] ?? null; } queue.push(...copy[i].children) } } return null;}</code></pre>

Simple BFS (modified level order traversal) solution | O(n)

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