Was testing your function and noticed that it gave me a bad access error, probably due to the recursion depth, when calculating factors(100_003). That said, I appreciate the way you thought of this recursively, very smart.

I agree with antong01. This problem is best solved in O(n) by not sorting either array, but instead making use of a dictionary. Most of the solutions submitted either use sorting which results in O(n log n) or searching through an array, which results in O(n^2) overall. While simple and concise to write, they don't scale as well.

Please mark your posts as having spoiler content even in Solutions when they have spoilers.

I suppose I should have incremented i in the else clause of my guard statement, as right now given an invalid character it would infinite loop.

guard let value = symbolValues[charArray[i]] else {
    i += 1
    continue
}