leetcode 235. Lowest Common Ancestor of a Binary Search Tree

Given a binary search tree (BST),find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

Given binary search tree:? root =?[6,2,8,4,7,9,null,3,5]

        _______6______
       /                  ___2__          ___8__
   /              /         0      _4       7       9
         /           3   5

Example 1:

Input: root,p = 2,q = 8
Output: 6 
Explanation: The LCA of nodes  and  is .
286

Example 2:

Input: root,q = 4
Output: 2
Explanation: The LCA of nodes  and  is,since a node can be a descendant of itself 
             according to the LCA definition.

242

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self,x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def lowestCommonAncestor(self,root,p,q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        node = root
        while node:
            if node.val in {p.val,q.val}:
                return node
            elif node.val > p.val and node.val > q.val:
                node = node.left
            elif node.val < p.val and node.val < q.val:
                node = node.right
            else:
                return node