Count valid BST subtrees
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`.
Implement
count_bst_subtrees(arr: list[int]) → intExamples
in
[[10,5,15,1,8,null,7]]out4What a strong answer looks like
State 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.
0:00 of about 30 min
solution.py
InputExpectedGot
[[10,5,15,1,8,null,7]]4not run yetsample