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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
The check for size == 0 is redundant. If the size is zero std::accumulate returns 0 anyway.
I think returning -1 isn't a good practice anymore. In general use std::optional for that.
In this case returning -1 is simply wrong! the sum of an empty vector just isn't -1, also suppose the vector was { 1, 1, -3 } then it's sum would also be -1.
A const is more or less just an int that represents an ascii value.
7097511 takes advantage of that by defining a new alphabet and then getting the corresponding letters.