let routingNumber = "026009593"

func isRoutingValid(routingNumber: String) -> Bool {
    let t = String(routingNumber.characters.filter { "1234567890".characters.contains($0) })
    let count = t.characters.count
    guard count == 9 else { return false }

    var n = 0
    
    // Run through each digit and calculate the total.
    for i in stride(from: 0, to: count, by: 3) {
        guard let a = Int(String(t[t.index(t.startIndex, offsetBy: i)])),
        let b = Int(String(t[t.index(t.startIndex, offsetBy: i+1)])),
            let c = Int(String(t[t.index(t.startIndex, offsetBy: i+2)])) else { return false }
        
        n += (a * 3) + (b * 7) + c
    }

    //print(n)
    
    // If the resulting sum is an even multiple of ten (but not zero),
    // the aba routing number is good.
    return n != 0 && n%10 == 0
}

isRoutingValid(routingNumber: routingNumber)