下面代码实现一个二叉排序树的插入函数（没有相同的数值），横线处应填入（ ）。
class TreeNode:
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
def insert(root, key):
if root is None:
return TreeNode(key)
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return rootif key < root.val:
root.left = insert(root.left, key)
elif key > root.val:
root.right = insert(root.right, key)if key > root.val:
root.left = insert(root.left, key)
elif key > root.val:
root.right = insert(root.right, key)if key < root.val:
root.left = insert(root.left, key)
elif key >= root.val:
root.right = insert(root.left, key)if key < root.val:
root.left = insert(root.right, key)
elif key > root.val:
root.right = insert(root.left, key)