# https://leetcode.com/problems/recover-a-tree-from-preorder-traversal

from typing import Optional

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

class Solution:
    def recoverFromPreorder(self, traversal: str) -> Optional[TreeNode]:
        i = 0
        def recurse(level, curr: TreeNode):
            nonlocal i
            val = []
            while i < len(traversal) and traversal[i] != '-':
                val.append(traversal[i])
                i += 1
            if len(val) > 0:
                curr.val = int(''.join(val))

            next_level = 0
            while i + next_level < len(traversal) and traversal[i + next_level] == '-':
                next_level += 1
            if next_level == level + 1:
                i += next_level
                curr.left = TreeNode()
                recurse(next_level, curr.left)

            next_level = 0
            while i + next_level < len(traversal) and traversal[i + next_level] == '-':
                next_level += 1
            if next_level == level + 1:
                i += next_level
                curr.right = TreeNode()
                recurse(next_level, curr.right)

        result = TreeNode()
        recurse(0, result)
        return result