divide and conquer 
class Solution(object):
    def maxSubArray(self, A):
        """
        :type nums: List[int]
        :rtype: int
        """
        
        def helper(A,left,right):
            if left==right: return A[left]
            middle=(left+right)/2
            leftmax=helper(A,left,middle)
            rightmax=helper(A,middle+1,right)
            leftsuf=A[middle]
            rightpref=A[middle+1]
            temp=0
            for i in reversed(range(left,middle+1)):
                temp+=A[i]
                if temp>leftsuf: leftsuf=temp
            temp=0
            for j in range(middle+1,right+1):
                temp+=A[j]
                if temp>rightpref: rightpref=temp
            return max(leftmax,rightmax,leftsuf+rightpref)
        return helper(A,0,len(A)-1)

class Solution(object):
    def maxSubArray(self, A):
        """
        :type nums: List[int]
        :rtype: int
        """
        local_max,global_max=0,0
        for x in A:
            local_max=max(x,x+local_max)
            global_max=max(global_max,local_max)
        return global_max