# 如果k作为参数传值，k -=1 并判断k == 0有问题，还是因为k本身是immutable
class Solution:
def __init__(self):
self.rank = 0
self.res = 0
def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
self.dfs(root, k)
return self.res
def dfs(self, root, k):
if not root:
return
self.dfs(root.left, k)
self.rank += 1
if self.rank == k:
self.res = root.val
return
self.dfs(root.right, k)
class Solution:
def __init__(self):
self.res = []
def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
self.dfs(root, k)
return self.res[-1]
def dfs(self, root, k):
if not root:
return
self.dfs(root.left, k)
if len(self.res) == k:
return
self.res.append(root.val)
self.dfs(root.right, k)