var zigzagLevelOrder = function (root) {
  if (!root) {
    return [];
  }

  const ans = [];
  const nodeQueue = [root];

  let isOrderLeft = true;

  while (nodeQueue.length) {
    let levelList = [];
    const size = nodeQueue.length;
    for (let i = 0; i < size; ++i) {
      const node = nodeQueue.shift();
      if (isOrderLeft) {
        levelList.push(node.val);
      } else {
        levelList.unshift(node.val);
      }
      if (node.left !== null) {
        nodeQueue.push(node.left);
      }
      if (node.right !== null) {
        nodeQueue.push(node.right);
      }
    }
    ans.push(levelList);
    isOrderLeft = !isOrderLeft;
  }

  return ans;
};

参考文献

链接：https://leetcode.cn/problems/binary-tree-zigzag-level-order-traversal/solutions/530400/er-cha-shu-de-ju-chi-xing-ceng-xu-bian-l-qsun/