如何在 def 中使用全局变量？

Question

Like so; the error is num is not defined

I want to use backtracking but how can I use a global variable in the funcation?

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 is out of scope. You have to define it before your inner function:

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

You are tring to evaluate something which is not declared before... How can i know if num is equal to 0 if you didn't even declare it?

Probably you missed a portion of function? By the way, the best solution is to declare it outside the def statment so the variable may be visible at the whole function.

Also remember to assign to it the correct value, num is in fact never used

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)

Did you mean:

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 has three types of variable scopes:

• Globals: variables defined outside a function body
• local: variables defined within a function body
• nonlocal: variables defined within a nested function

The scope is determined by where the variable is assigned, not by where it is used (read).

Function huisu has an assignment of num, so the variable is considered local to function huisu ie the line:

num = num - 1

Since it has not previously been given a value, the error occurs.

Two solutions

Solution 1

Declare num as nonlocal

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)

Solution 2

Pass num as a parameter (more typical solution)

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)