Calculate current sun using commutative sum

#1

How did we derive the equation
currentSum=cumSum[j]-cumSum[i-1] ?
And what is the case when i=0?
cumSum[-1]??

#2

hello @kartikaydwivedi55

handle i=0 case seprately . becuase for i=0 we cannot access i-1 (-1 index).

add this->

if(i==0) 
currentSum=cumSum[j]
else
currentSum=cumSum[j]-cumSum[i-1]

sum of element between (i,j) = sum of elements between (1,j) - sum of elements between (1,i-1)
note-> i and j r indices

#3

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