rotate.go

func rotate(nums []int, k int) {
    k %=len(nums)
	reverse(nums, 0, len(nums)-1)
	reverse(nums, 0, k-1)
	reverse(nums, k, len(nums)-1)

}

func reverse(nums []int, start, end int) {
	for i, j := start, end; i < j; i, j = i+1, j-1 {
		nums[i], nums[j] = nums[j], nums[i]
	}
}

rotate_old.go

func rotate(nums []int, k int)  {
    rand := make([]int, len(nums))
	copy(rand, nums)
	for k > 0 {
		rand = append(rand[len(rand)-1:], rand[:len(rand)-1]...)
		k--
	}

	for i, v := range rand {
		nums[i] = v
	}
}