如何在 def 中使用全局變量？

Question

像這樣； 錯誤是num未定義

我想使用回溯，但如何在函數中使用全局變量？

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        num = n
        def huisu(head):
            if head == None:
                return
            else:
                before = head
                head = head.next
                huisu(head)
                if num == 0:
                    head.next = head.next.next
                else:
                    num = num - 1
                head = before

            return head
        
        huisu(head)

Answer 1

num超出 scope。 您必須在您的內部 function 之前定義它：

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        num = 0 # or some other default value
        def huisu(head):
            if head == None:
                return
            else:
                before = head
                head = head.next
                huisu(head)
                if num == 0:
                    head.next = head.next.next
                else:
                    num = num - 1
                head = before

            return head
        
        num = n
        huisu(head)

Answer 2

您正在嘗試評估之前未聲明的內容...如果您甚至沒有聲明，我怎么知道 num 是否等於 0？

可能你錯過了 function 的一部分？ 順便說一句，最好的解決方案是在 def 語句之外聲明它，這樣變量就可以在整個 function 中可見。

還記得給它分配正確的值，實際上從未使用過 num

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        
        def huisu(head):
            if head == None:
                return
            else:
                before = head
                head = head.next
                huisu(head)
                if num == 0: # here
                    head.next = head.next.next
                else:
                    num = num - 1
                head = before

            return head
        
        num = n
        huisu(head)

你的意思：

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        num = n # Added
        def huisu(head):
            if head == None:
                return
            else:
                before = head
                head = head.next
                huisu(head)
                if num == 0: # here
                    head.next = head.next.next
                else:
                    num = num - 1
                head = before

            return head
        
        num = n
        huisu(head)

Answer 3

Python 具有三種類型的變量作用域：

• 全局變量：在 function 主體之外定義的變量
• local：在 function 體內定義的變量
• nonlocal：在嵌套的 function 中定義的變量

scope 取決於變量的分配位置，而不是使用（讀取）的位置。

Function huisu的賦值為 num，因此該變量被認為是 function huisu 的局部變量，即：

num = num - 1

由於之前沒有給它賦值，所以會發生錯誤。

兩種解決方案

解決方案 1

將 num 聲明為非本地

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        num = n             # Could assign num here

        def huisu(head):
            nonlocal num    # this declaration uses num in outer function
            if head == None:
                return
            else:
                before = head
                head = head.next
                huisu(head)
                if num == 0:
                    head.next = head.next.next
                else:
                    num = num - 1
                head = before

            return head
        
        num = n             # Or here, doesn't matter
                            # What matters is assigned before function call
        huisu(head)

解決方案 2

將 num 作為參數傳遞（更典型的解決方案）

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        
        def huisu(head, num):  # pass num as 2nd argument
         
            if head == None:
                return
            else:
                before = head
                head = head.next
                huisu(head)
                if num == 0:
                    head.next = head.next.next
                else:
                    num = num - 1
                head = before

            return head
        
        huisu(head, n)