Maximum Subarray Sum, including negative numbers.
Asked in the Round 1 (DSA + SQL + CS fundamentals) stage. Listed as "Maximum Subarray Sum, including negative numbers".
Implement max_subarray_sum(nums) to return the largest sum of any non-empty contiguous subarray.
nums is a non-empty list of integers. Return one integer. A subarray must contain at least one element.
def max_subarray_sum(nums):