代码思路：二叉搜索树中序遍历为递增序列。

class Solution:def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:def dfs(root):if not root:returndfs(root.left)if self.k == 0:returnself.k -= 1if self.k == 0:self.res = root.valdfs(root.right)self.k = kdfs(root)return self.res