find the balance index of an array where balanced index i is defined as the one whose left sum is equal to the right sum of the index .
i.e
summation (1 to i-1) = summation (i+1 to length of an array)
find the balance index of an array where balanced index i is defined as the one whose left sum is equal to the right sum of the index .
i.e
summation (1 to i-1) = summation (i+1 to length of an array)