Question
Given a binary tree as a level-order array `arr` (null for missing), return the number of subtrees that are themselves valid binary search trees (BSTs). A single node counts as a BST. A subtree means a node together with ALL of its descendants. BST validity uses strict inequalities (left subtree values strictly less than the node, right subtree values strictly greater). `0 <= node count <= 20000`.
count_bst_subtrees(arr: list[int]) → int[[10,5,15,1,8,null,7]]out4State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.
Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.