[tkzed49]

It's correct to omit constants from big O, but in the article W is a distinct input variable so the usage is valid. The size of the window W is bound by N, not a constant 26.

[ErikCorry]

But it's impossible for W to be bigger than 26 since you can't have 27 letters in a row that are all different.

Normally for a search algorithm the size of the alphabet is assumed to be constant.