116-populating-next-right-pointers-in-each-node.js

/**
 * // Definition for a Node.
 * function Node(val,left,right,next) {
 *    this.val = val;
 *    this.left = left;
 *    this.right = right;
 *    this.next = next;
 * };
 */
/**
 * @param {Node} root
 * @return {Node}
 */
const connect = function(root) {
  if (root == null) return null
  const cur = [root]
  while (cur.length) {
    let len = cur.length
    for (let i = 0; i < len; i++) {
      let el = cur.shift()
      if (i === len - 1) el.next = null
      else el.next = cur[0]

      if (el.left) cur.push(el.left)
      if (el.right) cur.push(el.right)
    }
  }
  return root
}

// another

const connect = function(root) {
  if (!root) return null
  if (root.left && root.right) {
    root.left.next = root.right
    root.right.next = root.next ? root.next.left : null
  }
  connect(root.left)
  connect(root.right)
  return root
}