# 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
import copy
class Solution:

    def bstFromPreorder(self, preorder: List[int]) -> Optional[TreeNode]:
        empty_tree = copy.copy(preorder)
        new_tree = [TreeNode(preorder[0])]
        preorder.pop(0)
        current = 0
        while len(preorder) != 0:
            new_tree = self.subTree(preorder, new_tree[current], new_tree)
            current += 1
        for i in range(0, len(new_tree)):
            if new_tree[i].val == -1:
                new_tree[i].val = 'null'
        return new_tree[0]
       
            
    def subTree(self, preorder,  x, new_tree):
        left = False
        right = False
        leftVal = TreeNode(-1)
        rightVal = TreeNode(-1)
        x_val = x.val
        if x_val != -1:
            for y in preorder:
                if left == False and x_val > y:
                    #left
                    left = True
                    leftVal = TreeNode(y)
                    

                if right == False and y >= x_val:
                    #right
                    right = True
                    rightVal = TreeNode(y)
       
            new_tree.append(leftVal)
            new_tree.append(rightVal)
            x.left = leftVal
            x.right = rightVal
            if left:
                preorder.pop(preorder.index(leftVal.val))
            if right:
                preorder.pop(preorder.index(rightVal.val))

            
        return new_tree