1. var averageOfLevels = function (root) {
    2.     if (!root) {
    3.         return []
    4.     }
    6.     const resList = [root.val],
    7.         countList = [1]
    9.     // row记录层数，并作为数组下标，相同层数的数据存在一起
    10.     const bfs = (node, row = 1) => {
    11.         let sum = 0
    12.         count = 0
    14.         if (!node || (!node.left && !node.right)) return
    16.         if (node.left) {
    17.             count++
    18.             sum += node.left.val
    19.         }
    21.         if (node.right) {
    22.             count++
    23.             sum += node.right.val
    24.         }
    26.         countList[row] = countList[row] ? countList[row] + count : count
    27.         resList[row] = resList[row] ? resList[row] + sum : sum
    29.         bfs(node.right, row + 1)
    30.         bfs(node.left, row + 1)
    31.     }
    33.     bfs(root)
    35.     for (let i = 0; i < resList.length; i++) {
    36.         resList[i] = resList[i] / countList[i]
    37.     }
    39.     return resList
    40. };