Given an integer array nums, find the subarray with the largest sum, and return its sum.

Example 1:
Input: nums = [-2,1,-3,4,-1,2,1,-5,4]
Output: 6
Explanation: The subarray [4,-1,2,1] has the largest sum 6.
Example 2:
Input: nums = [1]
Output: 1
Explanation: The subarray [1] has the largest sum 1.
Example 3:
Input: nums = [5,4,-1,7,8]
Output: 23
Explanation: The subarray [5,4,-1,7,8] has the largest sum 23.
解題思路
1.currentSum：紀錄目前子陣列的連續和。
2.若currentSum+nums[i]比nums[i]還小，表示前面部分沒幫助，重新開始。
3.maxSum：不斷記錄歷史最大值。