1class TreeNode:
2    def __init__(self, x):
3        self.val = x
4        self.left = None
5        self.right = None
6
7class Solution:
8    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
9        if not root:
10            return None
11
12        if p.val > root.val and q.val > root.val:
13            return self.lowestCommonAncestor(root.right, p, q)
14        elif p.val < root.val and q.val < root.val:
15            return self.lowestCommonAncestor(root.left, p, q)
16        else:
17            return root