Tell me about a time you worked on a coding task that had lots of numbers associated with it.
For the hands-on portion, implement min_subarray_len(target, nums), which returns the minimum length of a contiguous subarray whose sum is at least target. All values are positive integers. Return 0 if no qualifying subarray exists.
Input: a positive integer target and a list of positive integers nums. Output: an integer minimum length.
def min_subarray_len(target, nums):