Leetcode 2420. Find All Good Indices

Find indices i (k <= i < n-k) such that the k elements before i form a non-increasing sequence and the k elements after i form a non-decreasing sequence. Precompute lengths of non-increasing suffixes and non-decreasing prefixes (or equivalent) to test each candidate in O(1), yielding an overall O(n) time and O(n) space solution.