Hackerrank 'Separate the Numbers' Solution

separate_the_numbers.swift

func separateNumbers(s: String) -> Void {
    guard s.count > 1 else {
        print("NO")
        return
    }
    let s = Array(s)

    guard s.count > 2 else {
        let leftNum = s.first?.wholeNumberValue ?? 0
        let rightNum = s.last?.wholeNumberValue ?? 0
        let isAscending = leftNum > rightNum

        print(isAscending ? "YES \(leftNum)" : "NO")
        return
    }
    var substring = ""
    var lastAdded = -1

    for i in 0 ... s.count / 2 {
        substring = String(s[0...i])

        let num = Int(substring) ?? 0

        var validString = substring
        while validString.count < s.count {
            lastAdded = max(lastAdded, num)
            lastAdded += 1
            validString += String(lastAdded)
        }
        if validString == String(s) {
            print("YES \(substring)")
            return
        }
        lastAdded = -1
    }
    print("NO")
}