的Python：帶有鏈接節點

Question

實現優先級隊列時NoneType AttributeError的

class Node: 
    def __init__(self, data=None, priority='L', next=None): 
     self.data = data 
     self.priority = priority 
     self.next = next 

    def __str__(self): 
     return str(self.data) 

class P_Queue: 
    def __init__(self, head=None): 
     self.head = head 
     self.length = 0 

    def enqueue(self, node): 
     newNode = Node(node.data, node.priority) 
     if (self.head == None): 
      self.head = newNode 
     elif (self.head and self.head.priority == 'L' and newNode.priority == 'H'): 
      newNode.next = self.head 
      self.head = newNode 
     elif (self.head and self.head.priority == 'H' and newNode.priority == 'H'): 
      last = self.head 
      while (last.next and last.next.priority == 'H'): 
       last = last.next 
      if (last.next and last.next.next): 
       newNode.next = last.next.next 
      last.next = newNode 
     else: 
      last = self.head 
      while last.next: 
       last = last.next 
      last.next = newNode 
     self.length += 1 

    def dequeue(self): 
     node = self.head 
     print("next head: ") 
     print(self.head.next) 
     self.head = self.head.next 
     self.length = self.length - 1 
     return node 

    def is_empty(self): 
     return self.length==0 

def main(): 
    node0 = Node(0, 'L') 
    node1 = Node(1, 'H') 
    node2 = Node(2, 'H') 

    queue = P_Queue() 

    queue.enqueue(node0) 
    queue.enqueue(node1) 
    queue.enqueue(node2) 

    print(queue.dequeue()) 
    print(queue.dequeue()) 
    print(queue.dequeue()) 

main() 

所顯示的代碼在隊列（）while語句，我得到錯誤「的最後一行出現該問題的‘無類型’的對象有沒有屬性屬性（但是，根據我的print0語句（output：H）node0 = Node（0，'H'），我清楚地知道該屬性的值爲'H' （優先級），它不包含'None'值，所以對我來說只是令人難以置信的。

請幫助...如果有人有一個很好的資源，學習如何實現優先隊列與鏈接列表的初學者，這將是偉大的。非常感謝你，我在這裏死去。

下面回溯：

next head: 2 
1 
next head: None 
2 
next head: 
Traceback (most recent call last): 
    File "assignment1_3 queues.py", line 62, in <module> 
    main() 
    File "assignment1_3 queues.py", line 60, in main 
    print(queue.dequeue()) 
    File "assignment1_3 queues.py", line 39, in dequeue 
    print(self.head.next) 
AttributeError: 'NoneType' object has no attribute 'next' 


------------------ 
(program exited with code: 1) 

Press any key to continue . . . 

Answer 1

while循環的作品。您繼續轉發您的last = last.next，直到您達到NoneType。在您將last更改爲last.next之前，請確認其中有一個節點。我已經修改了你的這部分代碼：

elif (self.head.priority == 'H' and newNode.priority == 'H'): 
     last = self.head 
     print(self.head.priority) 
     print(last.priority) 
     while last.priority == 'H' and last.next: # <-- check last.next 
                 exists before pointing to it 
      last = last.next 
     if last.next and last.next.next: # <-- same thing here 
      newNode.next = last.next.next 
     last.next = newNode 

，這是輸出：

>>> main() 
H 
H