func combinationSum3(k int, n int) (ans [][]int) {
	stack := make([]int, 0)
	for i, sum, l := 1, 0, 0; ; l = len(stack) {
		// If need to backtrack
		if l == k || 10-i+l < k { // Cut the branches or MLE
			if l == 0 { // No previous number to pop
				break
			} else if l == k && sum == n {
				ans = append(ans, make([]int, k))
				copy(ans[len(ans)-1], stack)
			}
			sum -= stack[l-1]
			i = stack[l-1] + 1
			stack = stack[:l-1]
		} else {
			stack = append(stack, i)
			sum += i
			i++
		}
	}
	return ans
}