Sum of subtractions using Java Stream API -

everyone.

i've faced task, looked pretty simple me, spend 4 hours , left nothing. so, let's assume have simple list of integers

[1, 2, 3, 4, 5]

i want sum of subtractions each element - list, equal 4 -> (2 - 1) + (3 - 2)... , on.

to avoid answers range, list may this:

[5, 12, 16, 25, 25, 48]

it's growing, not same range between.

using simple for it's not hard like:

    int result = 0;     (int = 1; < lst.size(); i++) {         result += lst.get(i) - lst.get(i-1);     }

but i'm feeling myself utterly stupid when i'm trying solve case using streams.

is possible?

assuming have list of integers : [ n0, n1, n2, ... , nm, nn ], you're doing :

create list [(n1 - n0), (n2 - n1), (n3 - n2), ..., (nn - nm)].
sum list : x = (n1 - n0) + (n2 - n1) + (n3 - n2) + ... + (nn - nm). rigourously same (just swapped inside of parenthesises don't matter) x = (-n0) + (n1 - n1) + (n2 - n2) + ... + (nm - nm) + nn ].

which (as others pointed out) x = nn - n0. no matter inside list (except null).

thus, lst.get(lst.size()-1) - lst.get(0) is, far, best answer.

Insert