删除二叉排序树节点时，如果节点有两个孩子，则横线处应填入（ ），其中 findMax 和 findMin 分别为找树的最大值和最小值。

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

def find_min(node):
    """在二叉搜索树中找到最小节点"""
    current = node
    while current and current.left:
        current = current.left
    return current

def delete_node(root, key):
    if not root:
        return None
    if key < root.val:
        root.left = delete_node(root.left, key)
    elif key > root.val:
        root.right = delete_node(root.right, key)
    else:
        if not root.left:
            return root.right
        elif not root.right:
            return root.left
        else:
            temp = find_min(______)
            root.val = temp.val
            root.right = delete_node(root.right, temp.val)