235. Lowest Common Ancestor of a Binary Search Tree

题目：
https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

难度 : Easy

• 两个node，一个大于root，一个小于root，那么必定root两边，共同的ancestor是root，同时再考虑同为空的状况
• 两个node，都比node小，到左边去寻找，那么先找到那个必定是common ancestor
• 两个node，都比node大，类似….

AC解法

class Solution(object):
    def lowestCommonAncestor(self, root, p, q):
        """
        :type root: TreeNode
        :type p: TreeNode
        :type q: TreeNode
        :rtype: TreeNode
        """
        if root == None or root == p or root == q:
            return root
        elif p.val < root.val < q.val or q.val < root.val < p.val :
            return root
        elif p.val < root.val and q.val < root.val:
            return self.lowestCommonAncestor(root.left,p,q)
        else:
            return self.lowestCommonAncestor(root.right,p,q)