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: [-2,1,-3,4,-1,2,1,-5,4], Output: 6 Explanation: [4,-1,2,1] has the largest sum = 6.
Follow up:
If you have figured out the O(n) solution, try coding another solution using the divide and conquer approach, which is more subtle.
解题思路：O(n)算法，可以证明和为负数的前缀序列不会是最大和序列的前缀，因此当序列和小于0是则中断当前序列。
class Solution:
def maxSubArray(self, nums):
"""
:type nums: List[int]
:rtype: int
"""
ans, sum = -2147483647, 0
for num in nums:
sum += num
ans = max(sum, ans);
if sum <= 0:
sum = 0
return ans