```# 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 bstFromPreorder(self, preorder: List[int]) -> TreeNode:
        greater=[]
        smaller=[]
        if not preorder:
            return 0
        root_val=preorder[0]
        for i in range(1,len(preorder)):
            if preorder[i]<root_val:
                smaller.append(preorder[i])
            else:
                greater.append(preorder[i])
        root=TreeNode()
        root.val=root_val
        root.left=self.bstFromPreorder(smaller)
        root.right=self.bstFromPreorder(greater)
        return root