combination:
这个是典型的combination做法
dfs，注意中间结果一定要copy到新的变量中，不然会得到空值

var ans [][]int

func subsets(nums []int) [][]int {
    ans = [][]int{}
    cur := []int{}
    for i := 0; i <= len(nums); i++ {
        dfs(nums, i, 0, cur)
    }
    return ans
}
func dfs(nums []int, n, s int, cur []int) {
    if len(cur) == n {
//这里一定要拷贝
        tmp := make([]int, n)
        copy(tmp, cur)
        ans = append(ans, tmp)
        return
    }
    for i := s; i < len(nums); i++ {
        cur = append(cur, nums[i])
        dfs(nums, n, i+1, cur)
        cur = cur[:len(cur)-1]
    }
}