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 kthSmallest(root: TreeNode | null, k: number): number {
    16.     let queue: TreeNode[] = []
    17.     let order: number[] = []
    18.     while (true) {
    19.         while (root !== null) {
    20.             queue.push(root)
    21.             root = root.left
    22.         }
    23.         // if (queue.length === 0) {
    24.         //     console.log(order)
    25.         //     return 0
    26.         // }
    27.         const outQueueTreeNode = queue.pop()
    28.         if (!outQueueTreeNode) {
    29.             console.log(order)
    30.             return 0
    31.         }
    32.         order.push(outQueueTreeNode.val)
    33.         root = outQueueTreeNode.right
    34.     }
    35. };