Max Sum Subarray - Divide and Conquer

Question

I created a recursive function that takes in an array of ints, and returns the sum of the continuous subarray with the largest sum.

Example:

input: 1 4 -9 8 1 3 3 1 -1 -4 -6 2 8 19 -10 -11

subarray: 8 1 3 3 1 -1 -4 -6 2 8 19

sum: 34

My algorithm is a little off. About 2/3's of my test inputs are wrong. A list of my tests is below the code.

def max_sum_subarray(arr, left, right):

    maxLeftBorderSum = 0
    maxRightBorderSum = 0
    leftBorderSum = 0
    rightBorderSum = 0
    center = (left + right)/2

    if left == right:
        return arr[left]

    maxLeftSum = min_sum_subarray(arr, left, center)
    maxRightSum = min_sum_subarray(arr, center+1, right)

    for i in range(center, left, -1):
        leftBorderSum = leftBorderSum + arr[i]
        if leftBorderSum > maxLeftBorderSum:
            maxLeftBorderSum = leftBorderSum

    for i in range(center+1, right):
        rightBorderSum = rightBorderSum + arr[i]
        if rightBorderSum > maxRightBorderSum:
            maxRightBorderSum = rightBorderSum  

    return max(maxLeftBorderSum + maxRightBorderSum, max(maxRightSum, maxLeftSum))

Some tests:

1 4 -9 8 1 3 3 1 -1 -4 -6 2 8 19 -10 -11

Correct Answer = 34
My answer = 34

2 9 8 6 5 -11 9 -11 7 5 -1 -8 -3 7 -2

Correct Answer = 30
My answer = 28

10 -11 -1 -9 33 -45 23 24 -1 -7 -8 19

Correct Answer = 50
My answer = 47

31 -41 59 26 -53 58 97 -93 -23 84

Correct Answer = 187
My answer = 187

3 2 1 1 -8 1 1 2 3

Correct Answer = 7
My answer = 4

12 99 99 -99 -27 0 0 0 -3 10

Correct Answer = 210
My answer = 198

-2 1 -3 4 -1 2 1 -5 4

Correct Answer = 6
My answer = 6

Answer 1

Diffchecker

#!python3
def max_sum_subarray(arr, left, right):

maxLeftBorderSum = 0
maxRightBorderSum = 0
leftBorderSum = 0
rightBorderSum = 0
center = (left + right)//2

if left == right:
    if(arr[left]>0):return arr[left]
    else:return 0

maxLeftSum = max_sum_subarray(arr, left, center)
maxRightSum = max_sum_subarray(arr, center+1, right)

for i in range(center, left-1, -1):
    leftBorderSum = leftBorderSum + arr[i]
    if leftBorderSum > maxLeftBorderSum:
        maxLeftBorderSum = leftBorderSum
for i in range(center+1, right+1):
    rightBorderSum = rightBorderSum + arr[i]
    if rightBorderSum > maxRightBorderSum:
        maxRightBorderSum = rightBorderSum  

return max(maxLeftBorderSum + maxRightBorderSum,maxRightSum, maxLeftSum)

• The base condition is wrong
• for loop range mistake
• calling wrong function
• if python3 use true division