1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {void} Do not return anything, modify root in-place instead.
    12.  */
    13. var flatten = function (root) {
    14.   const list = []
    16.   const helper = (node) => {
    17.     if (!node) return
    19.     list.push(node)
    20.     helper(node.left)
    21.     helper(node.right)
    22.   }
    24.   helper(root)
    26.   for(let i = 1; i < list.length; i++) {
    27.     const pre = list[i - 1],
    28.           cur = list[i]
    30.     pre.right = cur
    31.     pre.left = null
    32.   }
    33. };