Easy

充分利用BST的properties.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null){
            return root;
        }
        if (p.val < root.val && q.val < root.val){
            return lowestCommonAncestor(root.left, p,q);
        }
        if (p.val > root.val && q.val > root.val){
            return lowestCommonAncestor(root.right, p,q);
        }
        return root;
    }
}