Find the length of the shortest subarray with a sum at least K. You may assume that the array has at least one element. You must solve this in O(n) time complexity.

``` def shortest_subarray(arr, K): from collections import deque n = len(arr) prefix_sum = [0] * (n + 1) for i in range(n): prefix_sum[i + 1] = prefix_sum[i] + arr[i] min_length = float('inf') q = deque() for i in range(n + 1): while q and prefix_sum[i] - prefix_sum[q[0]] >= K: min_length = min(min_length, i - q.popleft()) while q and prefix_sum[i] <= prefix_sum[q[-1]]: q.pop() q.append(i) return min_length if min_length != float('inf') else 0 ```