i did this first but i didn't i fked up the last part then later went with the best practice solution

Thanks for this.

You can use just "e", without +

Clever, and easy to understand.

Can you give examples of when they don't work?

Pure genius!

matter of habit

For Swift

let longRoutes = [["San Policarpo", "Oras"], ["Balangiga", "Lawaan"], ["Borongan", "Maydolong"], ["Jipapad", "Maslog"], ["Balangkayan", "Llorente"], ["Mercedes", "Guiuan"], ["Taft", "Sulat"], ["Sulat", "San Julian"], ["Arteche", "San Policarpo"], ["Oras", "Dolores"], ["Dolores", "Can-avid"], ["Can-avid", "Taft"], ["San Julian", "Borongan"], ["Maydolong", "Balangkayan"], ["Llorente", "Hernani"], ["Hernani", "General MacArthur"], ["General MacArthur", "Giporlos"], ["Giporlos", "Balangiga"], ["Lawaan", "Salcedo"], ["Salcedo", "Mercedes"], ["Maslog", "Arteche"]]

XCTAssertEqual(findRoutes(routes: longRoutes), "Jipapad, Maslog, Arteche, San Policarpo, Oras, Dolores, Can-avid, Taft, Sulat, San Julian, Borongan, Maydolong, Balangkayan, Llorente, Hernani, General MacArthur, Giporlos, Balangiga, Lawaan, Salcedo, Mercedes, Guiuan")

OH!

It does not necessarily and it is risky to assume so. They are returned in insertion order.

[Edit: to be precise, I did not found any insurance that going that way - with the constructor - will keep the insertion order]

Sorry. I forfeited eligibility and looked at the solutions. I see, issue removed.

Thanks!

Your code does not work properly when the number you are considering is a legitimate 0, in which case you just use the default value.

[I would also just use a .reduce() without using any mapping for performances - really no point in creating temporary variables here -, but that is as a side note.]