tree_to_linked_list.py

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


class Solution:
    def _helper(self, tree_root):
        this_node = Node(tree_root.val)

        if tree_root.left is not None:
            left_head, left_tail = self._helper(tree_root.left)
            head = left_head
            left_tail.right = this_node
            this_node.left = left_tail
        else:
            head = this_node

        if tree_root.right is not None:
            right_head, right_tail = self._helper(tree_root.right)
            tail = right_tail
            right_head.left = this_node
            this_node.right = right_head
        else:
            tail = this_node

        return head, tail

    def treeToDoublyList(self, root):
        head, tail = self._helper(root)
        head.left = tail
        tail.right = head
        return head