Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

package leetcode

func combine(n int, k int) [][]int {
	if n <= 0 || k <= 0 || k > n {
		return [][]int{}
	}
	c, res := []int{}, [][]int{}
	generateCombinations(n, k, 1, c, &res)
	return res
}

func generateCombinations(n, k, start int, c []int, res *[][]int) {
	if len(c) == k {
		b := make([]int, len(c))
		copy(b, c)
		*res = append(*res, b)
		return
	}
	// i will at most be n - (k - c.size()) + 1
	for i := start; i <= n-(k-len(c))+1; i++ {
		c = append(c, i)
		generateCombinations(n, k, i+1, c, res)
		c = c[:len(c)-1]
	}
	return
}