big integer multiplication and factorial

import Foundation

func mult(a: [UInt16], b: [UInt16]) -> [UInt16] {
    var c = [UInt16]()

    for (i, x) in b.reversed().enumerated() {
        var r = UInt16(0)

        for (j, y) in a.reversed().enumerated() {
            let k = i + j

            if k < c.count {
                r += c[k]
            }

            let z = x * y + r

            if k < c.count {
                c[k] = z % 10
            } else {
                c.append(z % 10)
            }

            r = z / 10
        }

        while r > 0 {
            c.append(r % 10)

            r = r/10
        }
    }

    return c.reversed()
}

func fact(n: Int) -> [UInt16] {
    var res: [UInt16] = [1]

    if n < 2 {
        return res
    }

    for x in 1...UInt16(n) {
        res = mult(a: res, b: [x])
    }

    return res
}