144.二叉树的前序遍历.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */

 var preorderTraversal = function(root) {
  let arr = []
  function preorder(root) {
      if (!root) return
      arr.push(root.val)
      preorder(root.left)
      preorder(root.right)
  }
  preorder(root)
  return arr

};