下面代码实现二叉搜索树的插入操作。假设树中不存在重复值，横线处应填写

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def insertNode(root, x):
    if not root:
        return TreeNode(x)
    if x < root.val:
        __________________________
    else:
        root.right = insertNode(root.right, x)
    return root