largest subarray sum using divide and conquer

the idea is: for each sub array we calculate 4 values in O(1) time based on the return values of its two halves. The meaning of the values:

l: the sum of the sub array with largest sum starting from the first
element
m: the sum of the sub array with largest sum
r: the sum of the sub array with largest sum ending at the last
element
s: the sum of the whole array
the recursive relation is clear in the code.