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。

 def lowestCommonAncestor(self, root, p, q):

         """

         :type root: TreeNode

         :type p: TreeNode

         :type q: TreeNode

         :rtype: TreeNode

         """

         if (p.val <= root.val and q.val >= root.val) or (p.val >= root.val and q.val <= root.val):

             return root

         elif p.val < root.val:

             return self.lowestCommonAncestor(root.left, p, q)

         else:

             return self.lowestCommonAncestor(root.right, p, q)