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 preorderTraversal(root: TreeNode | null): number[] {
    16.     const treeNodeList: number[] = []
    17.     const dfs = (root: TreeNode | null) => {
    18.         if (root === null) return root
    19.         treeNodeList.push(root.val)
    20.         dfs(root.left)
    21.         dfs(root.right)
    22.     }
    23.     dfs(root)
    24.     return treeNodeList
    25. };