This solution only works when p and q are in the tree, and for this problem, it seems that's the case.

```
class Solution(object):
def lowestCommonAncestor(self, root, p, q):
"""
:type root: TreeNode
:type p: TreeNode
:type q: TreeNode
:rtype: TreeNode
"""
if not root:
return None
if p==root or q==root:
return root
if p.val>q.val:
p,q=q,p
if p.val<root.val and q.val>root.val:
return root
if p.val>root.val:
return self.lowestCommonAncestor(root.right, p, q)
return self.lowestCommonAncestor(root.left, p, q)
```