func sortColors(nums []int) []int {
    // index is a colour (0, 1, 2)
    // value a is a frequency of the number in the parameter array
    colourFrequencies := make([]int, 3)
    for _, num := range nums {
        colourFrequencies[num]++
    }

    sortedIndex := 0

    // for each of the colours (0, 1, 2)
    for i := 0; i < 3; i++ {
        // iterate for frequency times of each number
        for j := 0; j < colourFrequencies[i]; j++ {
            // add the number to array
            nums[sortedIndex] = i
            
            sortedIndex++
        }
    }

    return nums
}

// func sortColors(nums []int)  {
// 	sort.Ints(nums)
// }