class Solution:
def __init__(self):
self.rank = 0
def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
ans = root.val
def inorder(root):
nonlocal ans
if not root:
return
inorder(root.left)
self.rank += 1
if self.rank == k:
ans = root.val
inorder(root.right)
inorder(root)
return ans
迭代（中序遍歷）
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
stack = []
while True:
while root:
stack.append(root)
root = root.left
root = stack.pop()
k -= 1
if not k:
return root.val
root = root.right