解题思路：

小的在左子树，大的在右子树。

class Solution {public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {if(root.val<p.val&&root.val<q.val)return lowestCommonAncestor(root.right,p,q);if(root.val>p.val&&root.val>q.val)return lowestCommonAncestor(root.left,p,q);return root;}
}