Flatten binary tree to linked list

leetcode

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def flatten(self, root: TreeNode) -> None:
        preorderList = list()
        stack = list()
        node = root

        while node or stack:
            while node:
                preorderList.append(node)
                stack.append(node)
                node = node.left
            node = stack.pop()
            node = node.right

        size = len(preorderList)
        for i in range(1, size):
            prev, curr = preorderList[i - 1], preorderList[i]
            prev.left = None
            prev.right = curr