Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum, and return its sum.

Example

Input:  nums = [-2,1,-3,4,-1,2,1,-5,4]
Output: 6
Explanation: [4,-1,2,1] has the largest sum = 6.

Hint: classic Kadane's algorithm — O(n) time, O(1) space.