Sum of All submatrix Approach-2

#1

While explaining in video bhaiya took an example of 1d array which was [1 2 3 4 5]. He used to add every element and update it then the result was [1,3 6,10, 15]. So I think that result was 15 . But he then subtracts (15-3+2) so result was 14.
I want to ask why we have done subtraction?
Even the sum is 15 but he told sum was 14 how??
And from where 3 and 2 came?

#2

@ashwani225
Please mention the point in the video where this took place

#3

Watch at point 3:00 in video of sum of all submatrix Approach -2

#4

@ashwani225
So the point was we need sum of elements from index 1 to 4
So we made prefix sum array
Then we did pre_sum[4] - pre_sum[1] + arr[1]
pre_sum[4] = 15
pre_sum[1] = 3
arr[1] = 2

So val is 14 which is sum of elements from index 1 to 4

#5

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