Stairs.swift

func stairs(n: Int) -> [[Int]] {
    if n == 0 {
        return []
    } else if n == 1 {
        let one = [[1]] + stairs(n - 1)
        let one2 = stairs(n - 1) + [[1]]
        return one + one2
    } else if n == 2 {
        let one = [[1]] + stairs(n - 1)
        let two = [[2]] + stairs(n - 2)
        let one2 = stairs(n - 1) + [[1]]
        let two2 = stairs(n - 2) + [[2]]
        return one + one2 + two + two2
    } else if n >= 3 {
        let one = [[1]] + stairs(n - 1)
        let two = [[2]] + stairs(n - 2)
        let thr = [[3]] + stairs(n - 3)
        let one2 = stairs(n - 1) + [[1]]
        let two2 = stairs(n - 2) + [[2]]
        let thr2 = stairs(n - 3) + [[3]]
        let result =  one + two
        let result2 = thr + one2
        let result3 = two2 + thr2
        return result + result2 + result3
    } else {
        return []
    }
}