首发于 语雀文档@blueju

    代码：

    1. /**
    2.  * Definition for a binary tree node.
    3.  * class TreeNode {
    4.  *     val: number
    5.  *     left: TreeNode | null
    6.  *     right: TreeNode | null
    7.  *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    8.  *         this.val = (val===undefined ? 0 : val)
    9.  *         this.left = (left===undefined ? null : left)
    10.  *         this.right = (right===undefined ? null : right)
    11.  *     }
    12.  * }
    13.  */
    15. function levelOrder(root: TreeNode | null): number[][] {
    16.     if (root === null) return []
    17.     /** 队列 */
    18.     let queue: TreeNode[] = []
    19.     /** 层序遍历 */
    20.     let levelOrder: number[][] = []
    21.     /** 添加第一层 */
    22.     queue.push(root)
    23.     levelOrder.push([root.val])
    24.     /** 当队列不为空时 */
    25.     while (queue.length !== 0) {
    26.         /** 记录当前队列长度 */
    27.         const currentLevelQueueLength = queue.length
    28.         /** 当前层队列 */
    29.         const currentLevelOrder: number[] = []
    30.         /** 遍历当前层节点，以此将节点出列，并将下层节点入列 */
    31.         for (let i = 0; i < currentLevelQueueLength; i++) {
    32.             const outQueueTreeNode = queue.shift()
    33.             if (outQueueTreeNode.left) {
    34.                 queue.push(outQueueTreeNode.left)
    35.                 currentLevelOrder.push(outQueueTreeNode.left.val)
    36.             }
    37.             if (outQueueTreeNode.right) {
    38.                 queue.push(outQueueTreeNode.right)
    39.                 currentLevelOrder.push(outQueueTreeNode.right.val)
    40.             }
    41.         }
    42.         /** 将当前节点队列加入层序遍历 */
    43.         currentLevelOrder.length && levelOrder.push(currentLevelOrder)
    44.     }
    45.     return levelOrder
    46. };