class Solution:

     def __init__(self):

         self.root = None

     def construct(self,order,root,i):

         if i==len(order):

             return None

         else:

             if self.root==None:

                 self.root = TreeNode(order[i])

                 self.construct(order,self.root,i+1)

             else:

                 if order[i]<root.val:

                     if root.left==None:

                         root.left = TreeNode(order[i])

                         self.construct(order,self.root,i+1)

                     else:

                         self.construct(order,root.left,i)

                 else:

                     if root.right==None:

                         root.right = TreeNode(order[i])

                         self.construct(order,self.root,i+1)

                     else:

                         self.construct(order,root.right,i)

     def bstFromPreorder(self, preorder: 'List[int]') -> 'TreeNode':

         self.construct(preorder,self.root,0)

         return self.root