You need the Maximum contiguous sum(Optimal Substructure):
def msum2(a):
bounds, s, t, j = (0,0), -float('infinity'), 0, 0
for i in range(len(a)):
t = t + a[i]
if t > s: bounds, s = (j, i+1), t
if t < 0: t, j = 0, i+1
return (s, bounds)
This is an example of dynamic programming and is O(N)