216. Combination Sum III

    func combinationSum3(_ k: Int, _ n: Int) -> [[Int]] {
        var res = [[Int]]()
        var list = [Int]()
        combination(k, 1, n, &list, &res)
        return res
    }

    func combination(_ count:Int, _ start:Int, _ remain: Int, _ list : inout [Int], _ res:inout [[Int]]){
        if list.count == count {
            if remain == 0 {
                res.append(list)
            }
            return
        }
        for i in stride(from: start, to: 10, by: 1){
            list.append(i)
            combination(count, i + 1, remain - i, &list, &res)
            list.removeLast()
        }

    }