如何删除具体的下级节点

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

def delete_node(root, key):
    if root is None:
        return root

    if key < root.key:
        root.left = delete_node(root.left, key)
    elif key > root.key:
        root.right = delete_node(root.right, key)
    else:
        if root.left is None:
            return root.right
        elif root.right is None:
            return root.left

        temp = find_min_value_node(root.right)
        root.key = temp.key
        root.right = delete_node(root.right, temp.key)

    return root

def find_min_value_node(node):
    current = node
    while(current.left is not None):
        current = current.left
    return current

# 示例用法
root = TreeNode(50)
root.left = TreeNode(30)
root.right = TreeNode(70)
root.left.left = TreeNode(20)
root.left.right = TreeNode(40)

# 删除节点 30
root = delete_node(root, 30)