二叉树锯齿形层次遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var zigzagLevelOrder = function (root) {
  if (!root) {
    return []
  }
  let queue = []
  queue.push(root)
  let order = true
  let res = []
  while (queue.length) {
    let n = queue.length
    let arr = []
    if (order) {
      while (n--) {
        let node = queue.shift()
        arr.push(node.val)
        if (node.left) queue.push(node.left)
        if (node.right) queue.push(node.right)
      }
    } else {
      while (n--) {
        let node = queue.pop()
        arr.push(node.val)
        if (node.right) queue.unshift(node.right)
        if (node.left) queue.unshift(node.left)
      }
    }
    res.push(arr)
    order = !order
  }
  return res
}