leetcode230 Kth Smallest Element in a BST

"""
Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.
Note:
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.
Example 1:
Input: root = [3,1,4,null,2], k = 1
   3
  / 
 1   4
  
   2
Output: 1
Example 2:
Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / 
     3   6
    / 
   2   4
  /
 1
Output: 3
"""
"""
用栈中序遍历，加一条判断语句
if len(res) == k
"""
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def kthSmallest(self, root, k):
        stack = []
        res = []
        cur = root
        while stack or cur:
            if cur:
                stack.append(cur)
                cur = cur.left
            else:
                cur = stack.pop()
                res.append(cur.val)
                if len(res) == k:  #中序遍历,中间加一个判断语句
                    return res[-1]
                cur = cur.right