Increasing Order Search Tree

increasing_order_search_tree.py

"""
Given the root of a binary search tree, rearrange the tree in in-order so that the leftmost node in the tree is now 
the root of the tree, and every node has no left child and only one right child. 

Example 1:
Input: root = [5,3,6,2,4,null,8,1,null,null,null,7,9]
Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

Example 2:
Input: root = [5,1,7]
Output: [1,null,5,null,7]
"""
# 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 __init__(self):
        self.arr = []
    
    def in_order(self, root: TreeNode) -> TreeNode:
        if root:
            self.in_order(root.left)
            self.arr.append(root.val)
            self.in_order(root.right)
            
    def increasingBST(self, root: TreeNode) -> TreeNode:
        sentinel = curr = TreeNode()
        self.in_order(root)
        for val in self.arr:
            curr.right = TreeNode(val)
            curr = curr.right
        return sentinel.right