Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. If there isn't one, return 0 instead.
For example, given the array [2,3,1,2,4,3]and s = 7,
the subarray [4,3] has the minimal length under the problem constraint.
Input: [2, 3, 1, 2, 4, 3], 7
Output: 2