附加此列表值后，为什么我的代码会停止

Question

当我运行代码时，在附加值9之后，它将继续停止（冻结）。在手动终止它并查看失败的地方之后。我还是不太明白。

该程序应能处理来自对象的列表。我使用了一个类List en和一个类Node。节点是帮助类。

任何见解都会非常感谢！

class List:
    class Node:
        """ Een node bevat een value (val) en een next-ptr (lnk) """

        def __init__(self, val, lnk=None):
            self.val = val
            self.lnk = lnk

    def __init__(self):
        self.root = None

    def toon(self):
        """ returns a string containing de elements-value of list.
            The order is de order in the list, values are seperated buy ","
            and enclosed by "[" and "]".
            An example "[een,twee,zes]".
        """

        string = "["
        printval = self.root
        while printval is not None:
            string += printval.val
            printval = printval.lnk
        string += "]"
        return string

    def append(self, val):
        """ Appends a node with value `val` to the end of the list.
        """
        node = self.Node(val)
        cur = self.root
        while cur.lnk != None:
            cur = cur.lnk
        cur.lnk = node

    def insert(self, val):
        """ Inserts a node with value `val` to the beginning of the list.
        """
        self.root = self.Node(val)

    def addSorted(self, val):
        """ Add a node with value `val` to list.
            It's place is based on value `val`.
            All the items in de list are sorted on their value'
            An example: We have "[een,zes]"
            Adding "twee" will give "[een,twee,zes]"
            To work well the list must be in order.
        """
        head = self.root

        # check if list is empty
        if head is None:
            node = self.Node(val)
            self.root = node
            return
        else:
            while head.lnk != None:
                # check if the value of root is greater than value
                if head.val > val:
                    node = self.Node(val)
                    node.lnk = head
                    self.root = node

                # if value smaller is than root value
                else:
                    node = self.Node(val)
                    node.lnk = head.lnk
                    head.next = node

        return

    def delete(self, val):
        """ Verwijder the node with value `val` from the list.
        """
        head = self.root

        while head is not None:
            if head.val == val:
                head.val = None
            head = head.lnk


if __name__ == '__main__':
    lst = List()          ; print(lst.toon())
    lst.insert("1")       ; print(lst.toon())
    lst.insert("0")       ; print(lst.toon())
    lst.addSorted("5")    ; print(lst.toon())
    lst.addSorted("3")    ; print(lst.toon())
    lst.addSorted("7")    ; print(lst.toon())
    lst.addSorted("4")    ; print(lst.toon())
    lst.append("8")       ; print(lst.toon())
    lst.append("9")       ; print(lst.toon())
    lst.addSorted("2")    ; print(lst.toon())
    lst.delete("2")       ; print(lst.toon())
    lst.delete("7")       ; print(lst.toon())
    lst.delete("9")       ; print(lst.toon())
    lst.delete("0")       ; print(lst.toon())
    lst.delete("11")      ; print(lst.toon())
    lst.append("9")       ; print(lst.toon())