text: ####  中序遍历——   左  ——  根 —— 右
  
  
  #####   Definition for a binary tree node.
  class TreeNode:
      def __init__(self, x):
          self.val = x
          self.left = None
          self.right = None
  
  class Solution:
      def inorderTraversal(self, root: TreeNode) -> List[int]:
          res = []
          self.inorder(root, res)
          return res
      def inorder(self,root,res):
          if root:
              self.inorder(root.left,res)
              res.append(root.val)
              self.inorder(root.right,res)
  
  
  
  
  ####   学习国际站别人的写法，列表与堆的方式
  # iteratively
  def inorderTraversal(self, root):
      res, stack = [], []
      while True:
          while root:
              stack.append(root)
              root = root.left
          if not stack:
              return res
          node = stack.pop()
          res.append(node.val)
          root = node.right