全局变量“res”在模块级别未定义

如何解决全局变量“res”在模块级别未定义

如何在作为类函数的函数中设置全局值？我知道我可以在构造函数中使用 self.res，但只是想知道为什么执行以下操作是错误的：

"""
 129. Sum Root to Leaf Numbers
 Given a binary tree containing digits from 0-9 only,each root-to-leaf path Could represent a number.
 Example:
 Input: [1,2,3]
    1
   / \
  2   3
 Output: 25
 Explanation:
 The root-to-leaf path 1->2 represents the number 12.
 The root-to-leaf path 1->3 represents the number 13.
 Therefore,sum = 12 + 13 = 25.
"""
class Solution:
    def sumNumbers(self,root: TreeNode) -> int:

        res = 0
        if not root:
            return res

        def helper(node: TreeNode,path: int):
            global res             <<<=============== ??? ERROR
            if not node.left and not node.right:
                res += path * 10 + node.val
                return
            if node.left:
                helper(node.left,path * 10 + node.val)
            if node.right:
                helper(node.right,path * 10 + node.val)

        helper(root,0)
        return res