230. Kth Smallest Element in a BST

class Solution(object):
def kthSmallest(self, root, k):
"""
:type root: TreeNode
:type k: int
:rtype: int
"""
self.result = 0
_ = self._tree_count(root, k)
return self.result

def _tree_count(self, root, k):
if not root:
return 0
left = self._tree_count(root.left, k)
if left + 1 == k:
self.result = root.val
right = self._tree_count(root.right, k - left - 1)
return left + right + 1