Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum.
Input: [-2,1,-3,4,-1,2,1,-5,4],
Output: 6
Explanation: [4,-1,2,1] has the largest sum = 6.
If you have figured out the O(n) solution, try coding another solution using the divide and conquer approach, which is more subtle.