Two Sum IV - Input is a BST (Leetcode 653) [Optimisation Required]

Problem Link: https://leetcode.com/problems/two-sum-iv-input-is-a-bst/

from collections import defaultdict

class Solution:
    def findTarget(self, root: Optional[TreeNode], k: int) -> bool:
        
        global d
        d=defaultdict(int)
        
        def fun(root,k):
            
            global d
            if(root is None):
                return False
            if(k-root.val in d):
                return True
            else:
                d[root.val]+=1
            l=fun(root.left,k)
            r=fun(root.right,k)
            if(l or r):
                return True
            else:
                return False
        
        return fun(root,k)