from collections import deque

#class TreeNode:
#  def __init__(self, val):
#    self.val = val
#    self.left, self.right = None, None


class Solution:
  def findSuccessor(self, root, key):
    if not root:
      return None
      
    queue = deque([root])
...

I might be missing something but I used deque instead of queue here