解法一：迭代法

  1. class Solution:
  2.     def inorderTraversal(self, root: TreeNode) -> List[int]:
  3.         ans, stack, cur = [], [], root
  4.         while cur or stack:
  5.             # 迭代至最左叶子，并将访问过的节点压栈
  6.             while cur:
  7.                 stack.append(cur)
  8.                 cur = cur.left
  9.             # 访问栈顶元素，然后将cur移至其右子树，对其右子树迭代
  10.             cur = stack.pop()
  11.             ans.append(cur.val)
  12.             cur = cur.right
  13.         return ans