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).”

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

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

通过观察可以发现，如果p和q分别位于root的两侧，那么root就是他们的LCA。否则的话，判断一下LCA应该是在左边还是在右边。注意L11 L13的return不能缺，因为这里并不是在调用这个函数，而是在递归定义。

 1 def lowestCommonAncestor(self, root, p, q):
 2         """
 3         :type root: TreeNode
 4         :type p: TreeNode
 5         :type q: TreeNode
 6         :rtype: TreeNode
 7         """
 8         if (p.val <= root.val and q.val >= root.val) or (p.val >= root.val and q.val <= root.val):
 9             return root
10         elif p.val < root.val:
11             return self.lowestCommonAncestor(root.left, p, q)
12         else:
13             return self.lowestCommonAncestor(root.right, p, q)

leetcode 235. Lowest Common Ancestor of a Binary Search Tree