Sum the Values of a Binary Search Tree That Fall Within an Inclusive Range

Given a binary search tree with distinct node values and two integers `low` and `high`, return the sum of every node value `v` with `low <= v <= high`. Return `0` if no value falls in the range. In a binary search tree, for every node, all values in its left subtree are smaller than the node's value and all values in its right subtree are larger. ### Function Signature ```python def range_sum_bst(tree: list[int | None], low: int, high: int) -> int: ``` ### Rules - **Input format.** `tree` is the level-order serialization of the tree. `tree[0]` is the root; after it, entries are consumed two at a time as the left child and then the right child of each non-null node, in the order those nodes appear. `None` marks a missing child, children of missing nodes are not listed, and trailing `None` entries may be omitted. - The input is guaranteed to be a valid binary search tree. - Both bounds are inclusive. ### Constraints - `1 <= number of nodes <= 20000` - `1 <= node value <= 100000`, and all values are distinct. - `1 <= low <= high <= 100000` - The result fits in a signed 32-bit integer. ### Examples **Example 1** Input: `tree = [10, 5, 15, 3, 7, None, 18]`, `low = 7`, `high = 15` Output: `32` Explanation: the values in range are 7, 10, and 15. **Example 2** Input: `tree = [10, 5, 15, 3, 7, 13, 18, 1, None, 6]`, `low = 6`, `high = 10` Output: `23` Explanation: the values in range are 6, 7, and 10. **Example 3** Input: `tree = [10, 5, 15]`, `low = 11`, `high = 14` Output: `0` Explanation: no value lies between 11 and 14.