Shortest Subarray with Sum at Least K

Given an integer array nums and an integer k, return the length of the shortest non-empty subarray with a sum of at least k. If no such subarray, return -1. Note: nums may contain negative numbers — sliding window alone won't work. Example 1: Input: nums = [1], k = 1 Output: 1 Example 2: Input: nums = [1,2], k = 4 Output: -1 Example 3: Input: nums = [2,-1,2], k = 3 Output: 3 DE Shaw Tier 1 — maps to LC 862. Optimal: Prefix sums + monotonic deque O(n). Build prefix sum array P. For each r, pop from front of deque while P[r] - P[deque.front] >= k (valid window, try to shrink). Push r to back after popping any indices with P value >= P[r] (they can never be better left endpoints).