$2.$ You have been given a list of integers, and your task is to find the maximum sum that can be obtained by selecting a contiguous subarray from the list. Write a Python function max$\_$subarray$\_$sum that takes in a list of integers and returns the maximum sum. For example, given the list $[1,-2,3,4,-1,2,1,-5,4],$ the contiguous subarray with the maximum sum is $[3,4,-1,2,1],$ and the maximum sum is $9.$