gistfile1.txt

import UIKit

func mergeStrings(a: String, b: String) -> String {
    var mergedString = ""
    let longString = a.count > b.count ? a : b
    let shortString = a == longString ? b : a
    for (indexShortString, character) in shortString.enumerated() {
        mergedString += "\(character)"
        let indexLongString = longString.index(longString.startIndex, offsetBy: indexShortString)
        mergedString += "\(longString[indexLongString])"
    }
    let remainingCharactersCount = longString.count - shortString.count
    print(remainingCharactersCount)
    let remainingCharactersIndex = longString.index(longString.endIndex, offsetBy: -remainingCharactersCount)
    return mergedString + longString[remainingCharactersIndex...]
}

mergeStrings(a: "abc", b: "12345678")