[Day42]-[回溯]- 组合

输入：n = 4, k = 2输出：[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

输入：n = 1, k = 1输出：[[1]]

func TestCombine(t *testing.T) {	var res [][]int	var trackBack func(n, k, start int, trace []int)	trackBack = func(n, k, start int, trace []int) {		if len(trace) == k {			res = append(res, append([]int{}, trace...))			return		}		for i := start; i <= n; i++ {			trace = append(trace, i)			trackBack(n, k, i+1, trace)			trace = trace[:len(trace)-1]		}	}	trackBack(4, 2, 1, []int{})	t.Log(res)}