public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) return null;
        if (root == p || root == q) return root;
        TreeNode l = lowestCommonAncestor(root.left, p, q);
        TreeNode r = lowestCommonAncestor(root.right, p, q);
        if (l != null && r != null) {
            return root;
        } else if (l != null) {
            return l;
        } else {
            return r;
        }
    }