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

    def __str__(self):
        return "\nValue: " + str(self.val) + "\nLeft:" + str(self.left) + "\nRight:" + str(self.right)

class Solution:
    def bstFromPreorder(self, preorder):
        
        if preorder is None or len(preorder) == 0:
            return None
        elif len(preorder) == 1:
            return TreeNode(preorder[0])
        
        root = TreeNode(preorder[0])
        
        # now collect the nodes that are less than root and make a tree from them
        leftChildrenItems = list(filter(lambda x: x < preorder[0], preorder[1:]))
        rightChildrenItems = list(filter(lambda x: x > preorder[0], preorder[1:])) # don't need >= because items are distinct
            
        root.left = self.bstFromPreorder(leftChildrenItems)
        root.right = self.bstFromPreorder(rightChildrenItems) 
        
        return root        

s = Solution()
print("Expected: [4, 2]")
print("Got:", s.bstFromPreorder([4, 2]))