398. Smallest Stable Index I

You are given an integer array nums of length n and an integer k. For each index i, define its instability score as max(nums[0..i]) - min(nums[i..n - 1]). In other words: - max(nums[0..i]) is the largest value among the elements from index 0 to index i. - min(nums[i..n - 1]) is the smallest value among the elements from index i to index n - 1. An index i is called stable if its instability score is less than or equal to k. Return the smallest stable index. If no such index exists, return -1.