Given an integer array nums, find the contiguous subarray (containing at least one number)
which has the largest sum and return its sum. A subarray is a contiguous part of an array.
Input: nums = [-2,1,-3,4,-1,2,1,-5,4] Output: 6 Explanation: [4,-1,2,1] has the largest sum = 6.
Input: nums = [1] Output: 1
Input: nums = [5,4,-1,7,8] Output: 23