Lowest Common Ancestor of a Binary Search Tree

Q:
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.
A:

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

其实这道题，无关Binary Search Tree ---虽然利用起来，效率会高一些些

public class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null || root == p || root == q)
            return root;
        TreeNode left = lowestCommonAncestor(root.left,p,q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);
        if(left == null)
            return right;
        if(right ==null)
            return left;
        return root;
    }
}

Mistakes: