# 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:

        def dfs(preorder,l,r):
            if l>=r:
                return None

            root=TreeNode(preorder[l])
            rightidx=l

            for i in range(l+1,r):
                if preorder[i]>preorder[l]:
                    rightidx=i
                    break

            root.left=dfs(preorder,l+1,rightidx)
            root.right=dfs(preorder,rightidx,r)
            return root

        return dfs(preorder,0,len(preorder))